i swear ive heard of a I6000D that used a X300 with HyperMemory, though i think its pretty rare. that could provide some insight, as that would already have HyperMemory support built into the BIOS. In fact, its possible that it would accept the X1400 without any problems.
either way, the biggest problem at the moment is that all the links to the BIOS dumps are dead, preventing anyone new from trying their hand at this. if someone could get those BIOS dump files back up, perhaps someone else could try?
heck, i know nothing of BIOS editing, but i wouldnt rule out trying it someday. it seems to me this is quite do-able based on what a dell engineer (supposedly) said (namely, that all the 6000D needs is the BIOS info for HyperMemory support). Or, if it was possible to disable HyperMemory on the X1400, that would probably work too.