I'm a US citizen currently in Mexico who needs to fly to London. I could avoid the US entirely by flying Air Canada. However, my trip would be shorter and easier if I chose an option that has me change planes in Houston or Miami. The challenge is that I would like to boycott US "security."

If I have a Mexico-LHR ticket, how likely am I to have to go through security in the US airport where I change planes (likely IAH or MIA)?

The details: All flights out of my current city go to the US first. To fly Air Canada, I would have to take a long bus ride to another Mexican city. I'd like to avoid the bus ride but I'd also like to boycott the TSA.