(accepting-values (*query-io*)
(setq foo (accept '(subset pick one or more of me)
:default foo :prompt "Pick")))
works for me in CLIM 1.0 on MCL 2.0b1p3, doing exactly what you asked for.
I assume you can also get this outside of accepting-values, using the
:view argument to accept, although I was unable to figure out how in the
two minutes I was willing to spend.