Just let your fingernails grow a little bit and you can get some really gnarly sounds, especially with tapes. No need for the pick.Even though I'm a pretty fair guitarist always using a pick these days, I have little facility with a pick on bass. That said, the few times I do grab one just to mess around, I'm reminded that there are some really cool sounds available that way that you're just not going to get with fingers. Somebody mentioned picking with a .73 and that's exactly what I've done with palm muting. Nice sound.
So yeah, play fretless with a pick, use a brass slide, whatever.