probably it is driven by a BridgeCo chipset, even according to their last year agreement: these chipset can host F/W, USB OTG, LCD display and other things, included ADAT or any multichannel I/O: at the moment they drive the majority of F/W audio interfaces out on the market, Terratec, Apogee X-Firewire, M-Audio, EgoSys, etc.
AFAIK and have seen, a working prototype of multi i/o F/W interface with USB-OTG does exist.
Apart from that (it doesn't add any practical vaue to the 3ad) i'm sure at 98% that CF slot can drive any IDE drive, since the pinouts are the same.
That 2% off depends on the format implementation by M-Audio.
This could help to build an external (self-battery powered) harddisk instead of those expensive and limited CF (BTW 2GB are enough for a long 24/96 stereo recording).
Schematics are around on the web, if anyone would try (me personally would like to try with a photocamera of a friend of mine - not mine of course
)