Yes unfortunately those parameters of within and whenT don't accept patterns.
You can promote make them patternable but it's a bit fiddly:
tidal> toPerc tpat f pat = innerJoin $ fmap (\t -> within (0, t) f pat) tpat
tidal> drawLine $ toPerc "<0 0.5>" (fast 2) $ "a b c d"
[11 cycles]
|abcd|abcdc-d-|abcd|abcdc-d-|abcd|abcdc-d-|abcd|abcdc-d-|abcd|abcdc-d-|abcd