Great, thanks! I tried something like that, but never had the space between the offset amount and following _, which in retrospect makes sense but I didn't think of at the time. I also agree in that context just the X-offset version looks better and now I'm using that in one of the two places it appears, though at the other some additional "collisions".
I've got hopefully just one more question for my current project before I declare it good enough -- I can't get positioning tweaks to apply to either \tempo indications or rehearsal \marks.
http://lilybin.com/vhf35h/15 shows that problem -- this one isn't really well-motived by the example and I just chose a random offset to show it not working (here I'm using X-offset but in context I probably want Y as well), but the output is the same whether the overrides are commented out or not.
Evan