Thanks for those tips!
@if_then_else, I have tried the ball-head solution, and it works really well. Part of my interest has been to keep things pretty compact & light, and when I add two ball-heads to all the other doodads & moving parts, it starts to feel like I've reached a tipping point in terms of size/weight/complexity. Might be that there's nothing for that, though... even if there was a jam nut that would fit the 3/8 hardware, I'm not sure I have enough thread on these adapters to use it effectively.
@voltronic I like the gopro mount idea, that's neat. I take this whole rig up and down a lot, so having a way to snap into predefined angles quickly would be handy. Can I change the pre-defined angles on that thing? I don't use XY a lot, and I like to experiment with ORTF & AB & other setups.
Later: Looking at my setup again now, I see that maybe I *can* use some 5/8 jam nuts, if I attach them to the 5/8-3/8 adapter threads that connect the clip/shockmount to the whole apparatus. I tried it just now with one 5/8 nut I had kicking around from another stand, and that actually works pretty well. Cool! Ridiculous problem solved, for now!