You can actually leave the path to p4 blank, and it will just use the PATH, I believe. I find that we rarely have to specify it explicitly (and in fact this will be going away entirely when the p4java version is completed)
A work-around for your issue would be to make the hostname:port for the proxies the same at all sites. If you can't set that up (hostname collisions etc.), then you can edit the 'hosts' file on the slaves to create a hostname that resolves to the appropriate server for each machine, then as long as you set up the port numbers to be the same on every proxy, then you should be good.
I think JENKINS-2947 is actually a good solution if we also allow node-level overrides for globally defined perforce servers. It would take quite a bit of work though, so don't expect a solution anytime soon. See what you can do about making your setup a bit more consistent across the board in the meantime.