I haven't been mountain biking in a while but if I were to get back on the mtb I'd probably just wear a pair of my regular padded lycra bike shorts underneath a pair of cargo shorts. Anyone who would have a problem with what you propose is nothing but an ignorant bike snob. I've been made fun of using a mountain bike bladder while road biking and was told "it's so mountain". Well, the only one with issues is the imbecile who said that. You do and wear what you want.