Wow, thanks for the detailed response, EYG!
See, I figured the answer would be something like that.... I wonder what it would sound like unbuffered, though. If it just resulted in the signal dropping out completely or dropping so far down it was practically useless, I'd chalk it up to real world constraints and walk away--probably the wiser course. Unless, of course, a simple micro buffer/mixer daughter-board would do the trick.
But
if the circuits in parallel and unbuffered actually created some cool broken amp, or otherwise interesting and strange tones and artifacts at sufficient volume not to trainwreck a song when I switch the pedal on, it still might be worth it to me. Of course I suppose it could just end up being 99% the sound of the loudest or lowest impedance fuzz; meh. It might be worth it to wire it up with clips just to hear it and without of course doing any drilling.
Also, there's a thread on TGP (
http://www.thegearpage.net/board/showthread.php?t=639632 ) about a fuzz user who had Kyle build him a custom Tweensome--an always-parallel circuit in which you used the volume controls of the Musket and Fix'd to control the degree of sonic mixing. I wonder if Kyle added a buffer/mixer to that circuit. He certainly knows his bizness, so my money sez yup. Don't know what Tag thinks about all that, though.
In any case, I'm already rethinking--I think I'd prefer to retain the original channel switching functionality on the right switch--I'm now wondering if I can configure a switch to select functions for the right footswitch--possibly one throw could be series with footswitch choice between musket>fix'd or fix'd>musket, center off could be original musket or fuzz function, and the final throw could be parallel in phase or parallel oop. Again, I realize what you're telling me, and those parallel options might turn out to be useless w/out onerous additional buffering/mixing, thus turning that option into too much hassle--mebbe I'd just go with a dt switch to choose b/w original and series "modes"; anyway I'm just spitballin' here and I appreciate you taking the time to school me some!

"In a moment of unparalleled genius, Noel Parachute headed off this potential disaster by unplugging the microphone."