I understand that you might want to have both pups on but only one of them effected, but this option is diminishing returns on tone IMHO. That said, if you really want it; best of luck. From a quick glance at the schem it looks like it might be possible without PCB hacking (if not I would discourage it more strongly - those old PCBs do not take to reworking very well) - I always figured it was a shared FX on/off, but it looks like comp and exp might be discretely triggered on that 3 way switch, via multiple poles.
Also, an idea re the 3 way FX switch - instead of replacing with 2 toggles (requires drilling at least 1 new hole in the body), consider replacing it with a single 4 way rotary switch (nothing, bright, fx, bright+fx) - like an EB varitone. No new holes required. A 2P4T would do it. If you also break out the comp vs exp this idea starts to get messy from a usability perspective, however, and you'd probably be better off with a 2 way bright mode switch and a 3 way fx switch in such a case.
Anyway, it is not a bad idea to do something here as that FX toggle is the most unreliable part in the whole bass though, I recently tracked down an intermittent signal problem to the long-buhing output jack (not a standard jack!). Mine is intermittently shorting tip to ring. This is rather unexpected as it's a quality Switchcraft jack, but I guess the cord got yanked a few too many times or something - just ordered/received a replacement and few spares - lucky I was ordering parts for a bunch of other projects anyway.
