I flew from Arizona to Vancouver BC in January, but being chicken left the plane at Bellingham and got a rental car. Returning to the US, the border guard queried the Colorado plates on the rental car and asked where I'd got it. When I said 'Bellingham' he got really curious about how I'd got there and a 15 min conversation about private flying ensued. To cap it all he said "why didn't I just fly into Canada illegally?".
After this, I went to the CBP office at Bellingham and asked them what the position really is. They didn't know and we had a long discussion about the pros and cons. It seemed they wouldn't normally quibble about somone turning up in these circumstances, but I was referred to a specialist officer who was out that day. I still have his phone number but never followed it up.
So I think the answer is, get a prior agreement from CBP at the port of entry or don't do it. They are perfectly approachable people although they may not say what you want to hear.