I happened to see broken MTB treks at the LBS, there were no mtbs when I worked at Trek other than some converted cruisers that were being ridden on dirt roads. Broken MTB frames at the seat tube/top tube is not uncommon, because they have long seat post exposure. Which is why you see extra tubes back there on a lot of bikes.

I like the way you think about the loads at the head tube. OTOH, the TT and DT are there to take up the moment. Certainly there are a lot of failures there, particularly on small frames with short head tubes. The problem at the seat cluster is that the seat tube will bend, and there is essentially a pivot set up by the stays and the top tube.