HME 900MHz units operate on 902.3 Base / 926.3 Headset (a.k.a. a + 24MHz offset). That's channel 1 and there's 16 channels in all. The channels are in 100kHz steps. The first 8 channels are for single lane drive thru's and the last 8 are for the second land in a 2-lane drive thru (Checker's/Rally's, some busier McDonalds come to mind).
http://www.hme.com/techPdfs/400434b.PDF3M 900MHz units also have 16 channels, 1st 8 designated for single lane, 2nd 8 for dual lane. They're on a 17MHz offset but I'm not sure which is the input and which is the output. Might have to check next time I'm out near a fast food joint that has them.
First Lane Channels:
Channel 1 = 903.2625 / 920.2625
Channel 2 = 903.5000 / 920.5000
Channel 3 = 903.7375 / 920.7375
Channel 4 = 903.9750 / 920.9750
Channel 5 = 904.2125 / 921.2125
Channel 6 = 904.4625 / 921.4625
Channel 7 = 904.7000 / 921.7000
Channel 8 = 904.9375 / 921.9375
Second Lane Channels:
Channel 1 = 906.1375 / 923.1375
Channel 2 = 906.3750 / 923.3750
Channel 3 = 906.6125 / 923.6125
Channel 4 = 906.8625 / 923.8625
Channel 5 = 907.1000 / 924.1000
Channel 6 = 907.3375 / 924.3375
Channel 7 = 907.5750 / 924.5750
Channel 8 = 907.8125 / 924.8125
I believe that ham radio has primary allocation of 902-928 so type 15 devices must accept all interference from licensed primary users
...although the band plan calls for ssb weak signal on the output and ATV on the inputs. In the states it's only a gentlemen's agreement and I doubt anyone would complain if you did some experimental wide FM stuff there haha...
You know, Radio Shack once sold these wireless headphones that the base transmitted WFM 900MHz and it was frequency agile with an analog knob. I think they operated around 912MHz though but I don't know just how frequency agile they are. The other option would be an analog STL, although they're quite expensive and not always frequency agile. You'd just have to get lucky finding a radio station closing up...