Sustainiac makes a Bass version of their Sustainer Driver which works extremely well - my next bass/"project" is going to be what I'm calling an "Effects Bass" and have that plus a Kahler Bass Tremelo so that played "clean" the Sustainiac sounds almost like a bowed cello, and with distortion and other effects sounds like that ... but infinitely sustained.

This sound like it would fit your needs, but *is* a permanent installment & modification to one bass only.