Just doing a follow-up with some time trial info. Did some testing at home today and ran the UA-5 & SVU-1 from one 9.6v R/C battery (see pic above).
UA-5 (phantom on, mics connected, RCA input from radio) and SVU-1 running the entire time. Was still working after 5 hours. JB3 was somewhere between 1 and 2 bars left. So in this case I think I'll just leave the SVU-1 on the whole time while connected to the jb3.
-john