Originally Posted by MikeSmoke

definitely possible to do...since we're not talking milliseconds, we're talking about a 15-20 second lag in the radio broadcast.
before the signal is beamed up, the feed arrives at the Sirius studio where a board op (or more likely, automation) inserts the Sirius promos, IDs, etc into the feed before it's sent to the uplink. so if you just insert a 15 second (or whatever the lag turns out to be) delay inline before uplinking the signal, mission accomplished.
it's the same way that local radio (if they're on the ball) synchs the radio broadcast with the local TV broadcast - there's always a lag there too because the feeds are sent via different satellites (or sometimes by other means, in the case of radio) back to the local studios before being transmitted. radio just inserts a delay. it's a pain in the ass to do, but is definitely very doable.