that, right there, is the stupidest thing i've ever seen anyone do with a seatpost/seat tube. the max height line is there for a reason, and i don't think a seatpost that short is designed to be used in a frame where the seat tube extends 3 inches above the top tube.

i think i saw a 3Rensho once with about a foot of seat tube above the top tube. it was also a TT "funny" bike. and also very stupid.

you hardly ever have to worry about a seatpost breaking under normal riding circumstances. even *gasp* a carbon post. the tubing of the frame is much thinner than the tubing used for a seatpost. they're basically indestructable, as long as they're being used properly (not inserted too shallow, not cutting a carbon post, not clamping some dumb QR rack to a carbon post, not exceeding the weight recommendations for said rack-even on an alloy post this is dumb-, the list goes on and on).