[jitter] Float32 "Value Keying" using a MOP?

Hi all
Here's something that's been coming up a few times in my latest project. Sorry if the answer's obvious, I just can't seem to find an elegant solution to this.
Basically what I'm looking for is an object / method of "keying" out only a certain range of values from a matrix, so that these are output to a smaller matrix containing only the relevant values. This is to be done for float32 data matrices, not video etc. I've currently done it on a cell by cell basis using jit.iter, but finding a MOP will be very nice indeed.
Any help is welcome!
-Nadav