Long gun carry in TX is pretty much unrestricted. You don't even need a CHL to do it. Open carry of long guns is the same, but expect the authorities to freak out because they're not familiar with it.
Technically, NFA stuff is illegal per state law, but the law gives you an affirmative defense against prosecution if you have it properly registered with the feds. Only really matters if it's noticed by a police officer that wants to make it an issue at all cost.
Non-standard containers are the way to go. Backpacks aren't all that uncommon these days, especially for the 20-something crowd. You can comfortably fit a 16" barrel bullpup like a MSAR STG-556 or (even more easily) a folding stock AK into a tennis racket case or book bag.
Loaded. With optics. Ready to roll. The only problem with it is time to deployment (and who carries a tennis racket around!?).
Doesn't 5.11 make some backpack that can carry an SBR sized rifle and be deployed rapidly? I seem to remember seeing it in the LAPolice gear catalogs. Can't remember what kins of weapon it was but probably a Sig556 or something like that because it allows for side folding...