Just thinking out loud ... would this be possible? Take the XL ROM from HTC (do they provide stock (stock Sense that is) images for their phones in the same way you can get them from Google for the Nexii?) and slap it onto the AT&T One X to get rid of whatever crap they put onto it, and even do the occasional free tether in a pinch? (Do the HTC ROMs have the stock android tether app?)
It might work. I know that's not much of an answer, but every One XL and the AT&T One X may not be identical. Different LTE frequencies may need different equipment, and there's a chance that we'll see several different versions of HTC One XL hardware, with the AT&T version matching some of it (Rogers in Canada comes to mind), or not matching any of it. Hopefully, this isn't the case, but until the "right" people get in there and dig around, the safe bet is to consider the possibility of two (or more) separate animals. Yeah, it can get confusing. The good news is that even of they are different, developers will be able to use bits and pieces of both. This is all hypothetical, as HTC really has no incentive to sell an unbranded, unlocked LTE phone.
HTC probably won't provide images for the One X or XL the way we see Google provide them. They aren't required to, and with so much proprietary software at play we don't see them bucking the trend any time soon. Chances are someone will be able to leak out RUUs, they seem to get them for every HTC phone, so there will be a "stock" image out there, albeit not necessarily factory authorized.
My best advice, without knowing how everything will play out, is to prepare to root and install custom software if you plan on changing things. You'll be voiding any warranty by swapping out to an XL image anyway, and this way you'll find exactly the options you're looking for. The AT&T version of the One X is bound to be a popular device, and hacking at it will follow shortly after release.