it is interesting it seems that the newer extremes actually have lost the ability, maybe it might be worth getting an older model y2flyy if you really need this?
eitherway though did you get it to work?
I think i have found a way too force the 5ghz mode, if you select the wireless options tab in the airport utility under wireless you can select a name for the 5ghz band. Then select that access point name in your airport wifi.
that's great, I get similar speeds myself it's a real step up especialy when transferring files around the network
Glad it worked out in the end and I'm sure this thread will help someone in the future
ps if you post your set-up in the forum, PM me because it'd be cool to see it after thinking about it for so long!
Dave