I actually think its a great idea, makes it more realisitc than having housecolor bands on units, you don't really see the M1A2 tanks out there covered in floresant paint, do you?
Hmm, you wouldn't need to do it via the hoard-effect
Look at the code for the Chem Suit, that... to is a TGA, but if you made a TGA for each house, it may be possible that way
But I really think that modifying the W3Ds would be the best way to go... in terms of complexity