Does this effect exist?

[Warren Sirota <wsirota@wsdesigns.com>]

I feel it would be useful to have a "freeze" effect that, while a footswitch was held down, would seamlessly loop the previous n milliseconds of sound (with n probably between 150 and 400 or so). This is perhaps for a solo jazz guitar context, but there certainly are other applications. So if i had a note that i was playing that I wanted to sustain while I drop in say, a couple of filler chords, i could hit the button and hold it down to do this (and, obviously, the chords would *not* be input to the freeze function). I don't know what you'd call this - "fingerless infinite sustain"? Anyway, maybe this is doable with one of the reverbs I have, or maybe there's some kind of dedicated effect. any ideas?

Thanks,
Warren