Hightower is sadly another cadet branch of the Tyrells- sorry.

But, the Tyrells of Brightwater Keep, the Tarlys, and everything else is available. I'm on my phone atm- I'll get back to you,  I would love to have a Reach vassal.

EDIT

Really quick, just to explain things:

I've got a lot of NPCs, some of which you could use, some of which, not so much, and Houses in the Reach that haven't even been taken up. There's the Redwynes of the Arbor, the Tarlys of Horn Hill. These have been named, but not really fleshed out much at all. The Tyrells of Brightwater Keep are still a good bet, as are the Shield Islands and most of the Northern Reach, which I haven't even looked at as of yet. Lord Owain of House Footly (at this point, he has been simply referred to as Lord Owain Tumbleton, to some confusion) is one such example of an NPC that I couldn't have touched. Sadly, the Hightower-Tyrell branch of the Tyrell family is something I want to work on myself- though if you really did want it, I could give you Lord Loras, who is someone I haven't really developed as of yet- you could definitely take his character. I could imagine that House Caswell, at the seat of Bitterbridge, could also be a good vassal house- they're at a key point on the Roseroad, and Garland is almost definitely, definitely going to be in contact with them soon, and haven't even been talked about one bit. 

PM me dude- I can help you get started, and figure something out :)