Hey dmaltby, I can't speak with authority to the custom remote side of your question, but I can definitively say that, currently, the controls within the app itself are not customizable to the extent that you can transfer functionality from one button to another.
So the short answer is, yep, you gotta switch to MISC to get that function. However - does your remote have any "soft keys" available? On my DISH remote, for instance, I can program some shortcuts to specific keys, or even learn certain functions from other remotes. My thought is that you could possibly re-program one of the DVR buttons to map to the "A" functionality.
Just a thought - this is likely a question for Alan Richey, the Remotes Whisperer, here on the forums...