well actually I was just gonna stick it on my MTB, currently got a profile Boa which is HUGE
any problem with doing this? MTB and BMX take a similar kind of beating, as long as it fits it should be fine, right? The only thing is it might affect the reach in a strange way
Modern BMXs have to take a hell of a lot more abuse than MTBs. Ok, the stem isn't that modern and it is only off a Haro complete, but it should hold up to anything you'll be doing on an MTB.
It will affect the reach because BMX stems are short (around 50mm). Also BMX stems are 22.2mm clamp so you won't be able to run regular 25.4 MTB bars.