Despite sharing the M5 bolts with the Avocet, I never heard of one breaking a bolt. Luck, or small numbers produced? Or maybe something about the leverage, from where they're positioned, they aren't stressed as much from your weight. I think (without a lot of analysis) that last bit is the main reason, but the first two might come into play as well.

When I made my first custom frame for myself in '77, I made it with a "seat mast" (though we didn't call it that back then) — seat tube of the frame went all the way up to the saddle-clamping hardware, no seatpost. I used the clamping hardware from a Weyless that someone had ruined the shaft on, taken from the trash bin. Worked great. Though nowadays I think seat mast frames are dumb, and I'm embarassed I ever made one! Hey, I was a teenager, my prefrontal cortex wasn't fully developed yet.