Does a high MTB seat post risk frame breaking On a full suspension MTB, does a high seat post (extending from frame by 25cm, 10" or more) contribute to frame weakness, should one look to have a bike with a tall seat tube, or are MTB's designed like this and shouldn't break?
There is usually a minimum insertion marker on the seat post to indicate the minimum amount of the seat post which should remain in the seat tube.  The frame manufacturer might also specify a minimum insertion depth for the frame.  It is not recommended to run the seat post inserted any less than the greater of these two this as it can put the frame under stress and you risk cracking the frame at the top of the seat tube (I've seen this happen first hand).

Having said that with a 400mm seat post I think you'd be able to have 250mm extending from the frame and the 150mm remaining in the seat tube would be sufficient.  You'd have to check the seatpost/frame recommendations but if I remember correctly the Rockshox Reverb dropper posts have an 80mm minimum insertion depth, so 150mm is certainly not extreme.