Just to follow up...
I decided to get an MMSI number through BoatUS, and it was very easy. I left the vessel information completely blank. As I learned via some on-line digging, I can update the account at any time (this is true for BoatUS, not necessarily for MMSIs obtained through other means). So in principle I could pop in the info for any charter boat at the time of the voyage.
The cost was $25 since I was a non-member. The number was assigned instantly, and I've now programmed it into my radio.
Dan <img src="http://www.traveltalkonline.com/forums/images/graemlins/Cheers.gif" alt="" />
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