Ok I will try to answer my own question as best as I can with known information.
(We assume some extra mass is not a problem in our design because we have a very cheap to launch coil gun.)
An intuitive way if simplified of understanding the issues would be a balloon filled with water:
- If it is stretched enough the walls of the balloon burst - either through support deformation or hydraulic/water weight pressure from within.
- If you hold it with little support the water pressure deforms it.
- Shock - if pressure, compression or g differences occur it could create heat and or shear forces.
- Weight differences between materials become more pronounced at high g - heavier plastic will sink down compressing the balloon to a flat plane at sufficient g even if the balloon is floating in water around it.
- Ambient pressure may affect some materials adversely. The plastic may melt, chemical reactions could take place or the water could change phase to solid ice.
None of 3-5 were issues during John Paul Stapp's tests - which I have now read in detail.
I have ranked 1-5 above in order of what would kill us (or a balloon first). Nr. 5 would not occur unless you made the balloon of styrofoam and/or reached incredible g. We can disregard that.
Nr. 4. will probably become a problem only at 50-150+ g as the human chest would cave into the lungs under its own weight as the air in the lungs offer no structural support.
Liquid suspension inside the lungs would help against this because it would reduce the mass difference between the lung space and ribs, probably pushing nr. 4 as an issue well above 100 g.
Bone density is 1.85g/cm^3 so water suspension would push this limit - whatever it is - up by 117% (1.85/0.85 = 217%). We know Strapp had no issues with this whatsoever at near 50 g so knowing nothing about bone strength/other similar issues I estimate this limit at 200-300g with lung liquid suspension and 100-150g without.
Nr. 3 I do not see as an issue. This is because you could build your coil gun to ramp acceleration slowly, cool the suspension fluid and so on. This and in general careful construction should prevent any shocks.
You might also construct the "launch bath" as a rotating structure so that it could always face the acceleration in the most optimal fashion.
Further about 20 cm of water would at the bottom at 100g only reach 2 bar additional pressure (20 meters of water gives 2 bar). This is quite negligible.
This brings us to nr. 2: Strapp actually cracked his ribs at around 18 g with some bad harnesses when facing the acceleration/deceleration. This makes a lot of sense as what looks like simple belts in his images would at this g have supported 1.4 tonnes if he weighed 80 kilos normally!
Turning the seat, better harnesses or in our case liquid suspension gets us well beyond 50g. If the liquid has the exact density of the human mass only internal differences in body mass/inability to compress evenly/negligible/without damage has consequences.
Finally the big killer: Nr. 1. Strapp had both whiteouts and redouts. The first being where blood left his eyes and the second where blood pressure burst every vein in his eyeballs.
Going back to the balloon example it would be like holding a heavy water filled balloon with nothing to support it - pressure would make the walls of it burst.
Water suspension may help here to some extent, but I don't know the limits: The eyeballs would be pressurized, most of the body mass, but the skull may prevent parts/all of the brain from being fully pressurized, as body fluids cannot move unrestricted past it and it will not itself flex - especially at rapid acceleration onset.
This low pressure at the back of the skull could mean veins bursting inside the brain - and any other body part not capable of "transmitting" pressure without tearing.
However we KNOW that water g-suits exist and work. These suits do not use hydraulics it seems, but the water itself. Our water suspension should work even better.
Further they are used in sitting position - a MUCH more vulnerable position to g force.
Since Strapp, as far as I can tell, used NO pressure suit and still took these amazing forces (25g for 1.1 seconds with a peak of 46.2g) and walked away unhurt (his eyes healed almost completely in few days time without surgery) I have to assume pressure assistance - whether from water suspension or a g-suit - would help greatly.
I think if you slowly go to high g from low g and let the pressure gradient created by the basin "settle" into the body's organs and fluids you could deal with this problem.
Upon leaving the muzzle of the coil gun however high pressure areas inside your body would suddenly have nothing pressing back - again creating bursting potential though less and of shorter duration. I would counter this by artificially increasing ambient pressure in the bath when leaving the muzzle to lessen the shock.
You could counter this by stopping the acceleration slowly, but that is not really an option with a realistic coil gun design.
With this in mind the pressure shock of going from 100g to 0g ie. 3 bar in the lowest parts of your body to 1 bar in the top of your body with no time to equalize is probably the limit.
Strapp experienced something like this at only 25g, but for a much longer duration - ie. constant ~0.5 bar pressure difference in his eyeball veins for 1.1 seconds with NO dynamic counteracting pressure.
So based on guesswork, these ramblings, the numbers above, the fact that Strapp was completely fine and died at 89 years old and that humans have been said to survive brief spikes of 1000g. I would say that breathable liquid suspension could allow astronauts to launch at 100g from a coil gun and be 99.9% okay after resting in a hyperbaric chamber/seeing a doctor.
Interestingly enough a 50 g acceleration to low earth orbit speed (7.8 km/s) would use a 62 km long launch tube, of which 1.9 km could be horizontal on the ground and the other 60 km would have to be in the air suspended with gas balloons. The launch tube would curve as the velocity vector changed and would exit at about 40-45 km height where atmospheric influence would be at 1% or less (exit heat shield still needed).
The launch would take 15.9 seconds.
If the accelerator is all/mostly on the ground and only maglev suspension inside the airtube the astronauts would face some g for longer to pay for cheaper construction costs. Not sure how much longer/at what g the in-air tube ride would be.