Clearly, I'm not Michael or Steve, but I am primarily a fretless player and an (aspriring) solo bassist.
I definitely agree about the expressive potential about fretless, and I'll also say that it's much closer to a voice than a fretted instrument is, so when you're playing, it can be very much like singing -- only you're not limited by natural gifts (or lackthereof).
If you've got the capacity (polyphonic tapping, looping like Steve) to do both bassline *and* a melody, you could, conceivably, be your own bass + "singing" duo. On
www.basssource.net , there's an interview with Steve where he's discussing with TB'er Tim Cole what frontiers of bass are yet to be probed, and he mentions that no one's really taken bass and singing very far yet. While it isn't technically singing, with the proper mixture of technique, note choice, and style, you *could* do this with a bit of work.
This is something I'm working on personally -- I'm in a program at school where <30 high school musicians spend a semester at my school and learn the broader points of recording, music theory, live performance, and making a living as a musician. I've got a few initial tracks done at the school and I'll be spending a lot of time with the Roland ADAT we've got (bands have the full studio booked solid) and post some examples within the next couple weeks of what I've been doing in this manner.