Not sure if anyone has tried this before, but here goes...

I have an existing CAT5 cable running between the downstairs and
upstairs of my house, length around 10 metres. Each end terminates on
a wallbox.

I'm now in a position where I would like to connect a downstairs USB
device (my Topfield PVR) to the PC upstairs.

The run length is too long for USB reliably, so I am considering a USB
CAT5 extender. I could run in another CAT5 cable for this, but it
would be a bit of a pain. HOWEVER, 10/100 BaseT network connections
only use 2 of the 4 available twisted pairs, hence I have 2 spare
pairs in my existing cable. It would be a trivial matter inside the
wallboxes to split these pairs off to a 2nd RJ45 module and send the
CAT 5 side of the extender modules down that. The question is, would
it work i.e. does USB over CAT5 cable only use 2 pairs in the same way
that real 10/100BaseT signals do?

These modules are quite common, but has anyone ever tried them with a
non fully wired CAT5 cable?