Scott - I have three suggestions for you - I have used all these methods
1: buy one of the sockets suggested above and slot it yourself; a dremel will do it with some patience.
2: locktite the pin at the proper torque and leave it alone long enough to set the locking compound (half an hour should do but you could wait overnight if you're worried); mark the pin and swing arm to show alignment, and then torque the locknut. I've done this multiple times and it has always worked.
3: borrow the windowed 30mm socket I have; I'm in Olympia and could mail it to you if you're not in a hurry, or meet you somewhere (though i5 is a bit of an issue what with that train derailment yesterday) if you are in a hurry.
Oh - if you are not in a hurry I know someone in Portland who makes them; I think a set (Allen and socket) is less than $50 but I'm not sure and he is a very busy guy running two other businesses (and being retired)...so that's four suggestions!
Edit to add this link:http://eazcycle.com/portfolio-items/30mm-swing-arm-socket/
No price listed but this is where mine came from; there is a chrome socket pictured, but the one of his I have is a black impact socket.