Perhaps filing a flight plan with a landing at a port of entry would be give them
the opportunity to deny entry before the flight. I assume you could cancel the
landing during the flight as soon as you were confident you'd get back to Canada,
and request a change by radio to a different port of entry if landing became
inevitable. Making minor(?) changes to a flight plan might be more acceptable to
CBP than landing without prior permission.

--

Many power pilots suggest exactly this. And it is legal, but you are going to seriously annoy Customs. They get a daily report with the incoming aviators, and they try to plan their day around it. This is not just my opinion, this feedback comes from the senior customs official in Great Falls. He would prefer to treat an emergency as an emergency, rather than see each overflight creating an operations exception. Hopefully he's not hoping to get a sweet deal on an Arcus at auction. :-)

My plan is to actually land and clear customs at least once. Now I'm in the system, I'm a familiar face. Hopefully if the worst comes to pass, they don't throw the book at me, just a minor paddling.

A lot of maybe's and wishful thinking going on here. If you have to land, it's a roll of the dice...

As an aside, there are airports right at the border. Del Bonita / Whetstone in particular would be perfect for clearing Customs on wave flights, but unfortunately it has been designed to kill glider pilots. Check out this picture -
http://www.aviationacres.com/FacilityPhotos/H28.jpg