Alan, about getting the time in a "decimal day" form: one way
to do this would be to hit Alt-T (or click on the time shown in the
legend area) to bring up the time dialog. You'll see that the time is
shown in the usual base-60 form at the top, but there's a "JD"
button partway down that shows the currently-set Julian Day,
including the decimal part of the day. The one thing to be careful
about here is that JD starts at noon, not midnight, so you'll
have to mentally add/subtract .5 days to the value shown.

I dunno how you're doing astrometry, but practically every piece
of astrometric software out there will do the math for you and will
produce an MPC report with everything properly formatted. As
Bob Elliott pointed out, Charon is among those programs:

http://www.projectpluto.com/charon.htm

When the Republican "decimalized" calendar was introduced in
France in 1793, there was also a decimalized clock with ten
hours/day, 100 minutes/hour, and 100 seconds/minute. It's a
pity this idea didn't catch on. (My guess would be that printing
up new calendars wasn't so big a deal, but there would have been
quite a few clocks out there built on the clumsy 24/60/60 system.)

Roger Pickard suggested that use of Tycho magnitudes might
not be the best way to go. I am somewhat agnostic on this point,
but in either case, it remains true that having Guide show stars
with a settable magnitude band (V, B, R, VT, BT) would be a Good
Thing. (And if I set it up to change bands at all, it will be quite
straightforward to allow for all of these bands.)

-- Bill