Yes it's a hack, but it's the hack used by bitcoind so it's sort of the standard (one could argue correct) way to represent the numbers.

Yes, poclbm should gracefully handle it. But also eloipool could handle it by serving out 1 in this special case and use pdiff for any higher figures. When sending the value to the client, just ceil(pdiff,1.0) it? Call it a workaround for poclbm or whatever. Or check client name for poclbm and only serve it to them.

The other option is that you and the poclbm author both refuse to provide a work around and then no one wins, but all poclbm users trying to mine at eloipool pools lose.