Well, I did the install last night, and everything is good. Hooked up the XM receiver, ran the antenna, and started writing the user app that will be controlling the receiver and displaying station/track info. I haven't actually subscribed to XM yet, but will do so as soon as I have enough control over the receiver to go through the activation process.
FWIW, the amp remote line draws 4 mA, so I didn't bother measuring what the XM box draws. I'm fine as long as it obeys the 500 mA specification. Thanks for your help, guys.