Have you read about UFO (United Functions and Objects) by John Sargeant, c.1992?  It wasn’t prototype-based, but your email otherwise brought it to mind.  There’s an overview paper here:
and several other papers can be found.

There was a spate of concurrent OO logic language papers in the late 80s/early 90s.  A skim of OOPSLA proceedings from that era should turn up many.  And before that there was ThingLab (in Smalltalk).