Sorry, this doesn't make any sense at all. Walls are not elements, they don't move, have any properties at all (no .tmp, no .temp, and no .ctype), and are just static things on a 4x4 grid. Since they have no ctype, then this really can't be done.

There are several other options you have though. Most involve using STOR. You can set STORs ctype and it will only eat particles of that type. Then you either spark some PSCN nearby to make it release the stored particle (Edit: Or ARAY works too and is really nice), or you have the STOR pass into PIPE/PPIP (or PRTI in the next version).