Perhaps a strobe light I remember that awesome thread here where somebody was creating a sub-bass sample instrument to be triggered via a guitar with a MIDI pickup - the samples were made on some absurdly long bench and a strobe light was used for tuning. Same concept as a strobe tuner - it will appear still when in tune.
That's pretty genius, actually. Now comes the problem of finding amps/cabinets to reproduce the sound. Maybe just set the bass itself up to another strobe light, so when you hit that low note it just strobes the stage. You could literally use the bass itself as a visual metronome for keeping the band in time. Who needs the drummer to count "one two three" when you can just hit a Mega-Low-F#?
Silly? Perhaps... Pointless? Perhaps... More so than tuning that low? Debatable.