The shuttle had a switch on panel C3 which allowed the crew to trigger a manual separation of the SRB. This switch had to have a purpose when moved from Auto to manual mode. What would have happened if the manual separation sequence would have been triggered at 70 seconds into the flight with the SRB at full thrust. Keep in mind that on the 51L flight both SRBs separated from the stack even without manual separation being triggered. Would a manual separation at full thrust have any likelihood of success?
The reason that the SRBs couldn't be "jettisoned" at any desired time is because the separation system as designed was not powerful enough to safely jettison the boosters if they were delivering a significant amount of thrust. The separation system simply severed the bolts holding the boosters to the External Tank and fired booster-mounted motors to provide clearance between them and the accelerating Orbiter/ET stack. If significant thrust loads had been present, then significant forces and moments would have been applied to the Orbiter/ET stack. It is possible that a more robust system could have been designed, but it wasn't incorporated in the system we had.
From my answer to this question (which see for references, etc).
Edit: the final report of the Rogers Commission on the 51-L accident explicitly states that the SRBs cannot be safely separated while producing thrust: (italics mine)
- The Space Shuttle System was not designed to survive a failure of the Solid Rocket Boosters.
There are no corrective actions that can be taken if the boosters do not operate properly after ignition, i.e., there is no ability to separate an Orbiter safely from thrusting boosters and no ability for the crew to escape the vehicle during first-stage ascent.
Reference: Rogers Commission Report, page 187.
The SRB separation sequence is designed to be used after thrust drops off enough that the SRBs will fall behind the orbiter. The separation rockets push the SRB nose out. If you separated the SRB while at full thrust, you'd expose the orbiter and ET to the exhaust of the SRB. At hundreds of tons of thrust, that's a lot of force.
The aft attachment points consist of 3 cylinders with "pistons", all hinged in a plane perpendicular to the thrust vector. At full thrust, the pistons would be wedged into the cylinders with a large force. This force may be enough to prevent separation.
I haven't found a NASA study on early separation. The standard abort modes have the SRBs attached to the vehicle until burnout.
If an event requiring an abort happened after SRB ignition, it was not possible to begin the abort until after SRB burnout and separation about two minutes after launch.