Well, this is interesting -- and encouraging.
I set up Pidgin for an eBox Jabber account on a computer I use, where Pidgin is already in use. Then I installed Pidgin on another computer, and set up another user. Both are XP machines, both are using the same (current) version of Pidgin.
Here's the interesting part. I can see "me" on the other computer in a perfectly normal fashion; online, just as normal. On my computer, the test user appears off line and "not authorized." But it's possible to IM between both machines. So it does work, but not exactly as one would expect if for no other reason than that Pidgin (or Windows) is behaving differently on a per machine basis for the eBox Jabber account only.
So progress is being made. Now if I can sort out the off line/not authorized thing (which will just confuse people on the network), I'm home!
Thanks for everybody's patience and helpfulness.