Try the .p <name> and send them a pm That works unless you've been squelched.
That's what I've been doing but, nine times out of ten, I don't get any response. Sometimes I think that's because the person doesn't know how to respond to IMs. Sometimes I think it's because the person actually isn't online, even though the roster shows him to be there. I've noticed if I'm the only one on the roster, it nearly always shows one person still in the AvA arena after I log out.
As an aside, I did use the .p to actually accomplish something a couple days ago.
I noticed one guy was logged in as a bishop for some time. Knights and Rooks were the usable teams. I IM'd the guy and told him to switch to Rooks if he wanted to play in the AvA. He actually responded, thanking me for letting him know.
New guys are often confused (as I was) that there are only two functioning teams in AvA.