Hmmm, it looks like Hit Points are being limited using the maxscore() tag, which I don't think can be targeted with bonuses.
So, it looks like you'll have to have Mountain of Meat include a replacetags() to change that tag on Hit Points to the correct value. Yeah, Barbarians already does that to set it to 50% instead of 30%.
Code:
replacetags(in "ST:Hit Points" with "maxscore(ST:ST * 2)")
The replacetags() won't be undone automatically, unfortunately, if that trait is removed later, so there's a caveat.