I have what seems to be a more road-tuned crossover. Being not a mountain biker, I'm not sure how this would go, but ...

What if you got a nearly-mountain hybrid, one that is "a mountain bike" but if you slap slicks on it it's decent enough for a city commute?

Then slap some city tires on, ride to the trail, get off and deflate. Swap out for your mountain bike tires, raise your handlebars, fold up the road tires and slip 'em into your CamelBak.