There's already a half assed HD ROM out for the regular Desire:
[09-22] newSense DesireHD ROM(1.15) file for Desire phone, rooted|A2SD+|OJWake|OVPN - xda-developers
The camera doesn't seem to work though (8MP on HD vs 5MP on regular).
I would think a ROM ported from Desire Z would be more suitable because they both have the same screen size and Camera resolution. The keyboard support can also be stripped out of the ROM.
Moreover, the Z supports HTC Sense in landscape mode, which is pretty cool. I don't think HD supports this feature.