That's not a bad idea, a little tricky maybe but certainly not bad

I take it you've spent some time looking over the schematics, as you say the fact that you couldn't use both the MMCmobile and the miniSD at the same time wouldn't matter if you're looking at using 4-8GB miniSDHC cards; the ability to use MMCmobile would almost become redundant since you're doubling the device's capacity at the very least!

Do you have any idea as to when you'll start dissecting your 770?