It is indeed a Catch 22 for Clemson, as you correctly point out Texas' control of the Big 12. In my opinion, Clemson would do best to stay in the ACC and ride out this whole thing, UNLESS it becomes apparent that the ACC will become completely irrelevant on the national stage. And while the ACC is essentially a non-player on national stage as it is (outside of FSU/Miami, if they rise again), I find it hard to believe that they will drop to the level of MWC, Conf USA, Sun Belt, WAC, etc. in football. There are just too many big-name schools in the ACC for that to happen, especially with the recent additions of Pitt and Syracuse.
I think the worst thing Clemson can do is to make a knee-jerk decision and bolt for the Big 12 (if offered), unless it becomes a necessity at some point. While the Big 12 sounds like heaven to Tiger fans right now, they'd ultimately be trading Tobacco Road cronies for Longhorn Nation's power. Make no mistake about it, Texas (and OU to a lesser extent) runs that conference and always will. Just look at how the conference's money has been "divided" in recent years. Beyond the Texas issue, travel will be an absolute nightmare and money pit, especially for the non-revenue sports. Plus, Clemson would be throwing away decades' worth of rivalries overnight (i.e. NCSU, GT, etc.) and wouldn't have a conference member anywhere close by.
Assuming Clemson doesn't get an SEC invite somewhere down the road, we (USC) still come out on top in the big picture. Thank God we joined the SEC back in 1991!