The one button is not a problem, you can wire all three of the homelink buttons or just one - it doesn't matter. The 9V supply is a little more problematic but if you were enthusiastic enough you can just wire in a simple 9V regulator. Its just one tiny component that will take your 12v supply from the car and give you 9V to power the remote - still its a little more fiddling around.

good luck,
Seedar.