You sure about that? Not that I know what you mean when you say "...would be like plugging the Sirius antenna cable into the headset jack of a Sportster."
The sirf III receiver/antenna is a good upgrade for older systems, plug a sirf III into the E46/E39 system and it locks onto a signal in seconds, plus you get full satellite fixes in an multi story car park, compare that with the factory antenna where you can wait up to 2 minutes for a fix and even driving through urban areas with tall buildings can be enough to loose the sat fix.
I think because there is a speed pulse, car makers tend to put a pretty poor oem antenna in. Maybe it is getting better now?? But the nav in my E91 was a bit slow compared to my Sony nav system, and again while the Sony works in the garage the BMW system looses its satellite fix.
So I see no reason to try and use the factory antenna myself.