I'm one of those who reported driving to the border, getting a Visa Waiver, driving back to the plane and flying into the US via a port of entry (Bellingham). All this was new to my friends in the Pacific School of Flying who had apparently never before encountered the issue of a UK national flying a Canadian aircraft in the US.
But that was 12 years ago. In any case it was only neccessary since I had travelled initially to Canada, not the US. If I had travelled first to the US, and the driven to Canada, I would already have had a Visa waiver.
Since then the US has introduced ESTA and there is no more visa waiver. Esta authorises you to enter the US and doesn't appear to specify how, but the problem in 2000 was that there were no facilities to issue visa waivers at ports of entry which is why you had to get it at the border. If you already had one, it was valid even at Bellingham. With Esta it may be different because the 'travel authorisation' is issued before you leave London. A phone call to US customs at Bellingham might clarify all this.
Be aware that if you plan to fly a US aircraft into Canada then the plane will need to be insured for Canada (Most insurances specify the 48 contiguous states) and it will need to be stickered for flight in Canada. This could involve some expense for your FBO.
And then there's eApis and Canpass of which I have no experience, but may well fall over when you can't provide a zip code or social security number. I'd be very pleased to hear more about this from anyone who has done it recently since I was discussing the very issue with my own FBO earlier this week. In a recent trip I got to Astoria, Oregon, and would have continued the further 150 mi to Victoria but for this bureaucratic quagmire.