This would be an awfully tough antitrust case.
Here's the problem: According to the U.S. Supreme Court, NCAA football is "an industry in which horizontal restraints on competition are essential if the product is to be available at all." -NCAA v. Board of Regents, 468 U.S. 85, at 101 (1979).
UConn would have to do a lot more than point to the existence of various horizontal restraints that cause harm as to UConn. Behavior that would, in other industries, be condemned as per se illegal is subject to the rule of reason in the sports context. Shouting "collusion" or "group boycott" is not going to get you anywhere. Every conference is, by definition, a group boycott of other teams. And members of every conference must meet and agree on schedules and rules that all will follow.
To successfully challenge the P5, UConn or another plaintiff would have to demonstrate that a particular restraint or group of restraints is unreasonable -- which would mean demonstrating that they have the effect of (or the clear purpose of) restricting the supply of top tier college football contests. (This would involve defining "top tier" P5 football as a separate product market from G5 football, which while possible would nonetheless be an uphill battle.)
Technically, a plaintiff could succeed just by demonstrating that the effect of the P5 agreements is to restrict the supply of "top tier" football -- but that would involve forecasting the future, which a court would be reluctant to do in such a complex industry. Thus, as a practical matter, a suit is unlikely to be successful without clear evidence of anticompetitive intent (e.g. "starving" non P5 schools of money needed to compete at a "top tier" level.) I would not be at all surprised if this is, in fact, the motivating force behind much of what the P5 is doing -- but finding sufficient evidence to demonstrate that it was a conscious plan or scheme (and not just something one particular AD wrote in an internal email) would be extremely difficult.
