Sun-Jupiter L1 is 1/3 of an AU from Jupiter. Could it be used to slow down or redirect a spacecraft approaching Jupiter? And the same for Sun-Jupiter L4 and L5 for a spacecraft on the way to e.g. Saturn?
The Sun-Jupiter L1 and L2 (SJL1 and SJL2) are lot more interesting than the Sun-Earth L1 and L2 (SEL1 and SEL2). The Sun-Earth mass parameter is only 3.04e-6 while the Sun-Jupiter mass parameter is 9.54e-4. The Weak Stability Boundaries (WSBs) emanating from SJL1 and SJL2 are a lot more extensive than the WSBs from SEL1 and SEL2.
Comet Oterma enjoys some fame due to it's occasional transits through SJL1 and SJL2. Sometimes Oterma orbits beyond Jupiter's 5.2 A.U. orbit. Then it falls through SJL2 and leaves Jupiters' realm via the SJL1 neck. Here's a screen capture from a pdf by Koon, Lo, Marsden & Ross:
It's interesting that in a rotating frame, Oterma's inside orbit looks a lot like a Hilda Comet but rotated 60º. The Hildas have a 3:2 resonance but their aphelions take them to the L4, L5 and L3 regions far from Jupiter's influence. Oterma's third aphelion brings it to SJL1, close enough that Jupiter is a big influence.
To answer your question, is the SJL1 useful for fly-by probes? In my opinion, no. The arrival Vinf for an earth to Jupiter ellipse is around 5.6 km/s. Too high for ballistic capture. SJL1 orbits a little slower than Jupiter, but this delta V savings is minor compared to the substantial Oberth benefit Jupiter's deep gravity well can offer.