OK, still haven't been getting any come through so I've done some more research and come up with something curious.
The proxyper manual was what I quoted from before, and it clearly says to use "blocksize" to specify the block size and that the default is 256. However, in the included example proxyper.ini it has the option "requestsize=256" in the RC5-72 section. I checked the latest FreeBSD and Linux tarballs and it was apparent for both.
Looks like a mix up on their part. I've changed my proxyper to include both blocksize=1024 and requestsize=1024 and there were no warnings about unknown options when I restarted. Please can you do the same?
edit: Looks good. My proxyper just hit the FDC one up for some more blocks and I saw "[2010-02-10 12:49:52] server: Queueing rc5-72 request for iter 1024."