I put together a little circuit last night that lights red when the battery falls below a set level - it has a mini trim pot so you can set the trigger voltage where you wish.
[url=http://www.flickr.com/photos/alheeley/6525394351/][/url]
I just want to take this one stage further to have a bicolour LED that stays green when the battery is above 7.5V, then goes red when below 7.5 to show the battery is running out. The 7.5 is my arbitrary level, you can set this at whatever is appropriate - just wait till your preamp sounds like its starting to break up, then check the voltage and set the circuit accordingly. One single bicolour LEd would be very unobtrusive on a bass body, and the current drain should be minimal.
All I have to do now is figure out how to do it. Zener diode route is another (passive) option, but not fully figured that one yet.