Doh! I'm already using the whole width of the neck as well as its length (mine is an ordinary four-string bass), but your comment reminded me of something I really should have thought of earlier: if I go to dropped-D tuning, that has the effect of giving me two more frets of range after I transpose everything down a step. I should have remembered that trick on my own because I used it to make a Bach organ piece fit onto a regular guitar several years ago.
(no subject)
Thanks for the reminder.