AJ,
Here's a "workaround" for your bass delema.
Write a custom sound with a split bass as part of the main sound. Maybe even have it layered the whole length of the keyboard. That way, the sustain will affect the bass part too.
Personally - I can't play a polyphonic bass pasrt WITH sustain, because it gets all gummed up, but maybe you have a different style.
Try writing a custom sound with bass as one element of the sound .... it should work !