Thanks to all for the replies.
I'll post the following in case someone down the road finds this thread with the same question.
I called Transport Canada as well as Canada Border Services today.
It's amazingly easy; essentially, they just want to see a bill of sale
at the border, and make sure the trailer is properly registered (didn't really seemed concerned about the boat).
The trailer should be registered either in Yukon with the former owner, or presently in Alaska with a temporary tag registered to me - either one would work.
It's also possible, though very unlikely they said, that the border agents will want an 'affidavit of transit through Canada only' document signed, a promise that neither the driver nor his truck will be staying in Canada - the fee for the document would be 1/60th the value of the load.
Very nice people up there to deal with.
I also have a call/message into U.S. Customs and Border Protection today; haven't called me back yet. I'd like to make sure I'm square on their end as well.
I'll do a follow-up here when they call.