First off, obviously this agreement wouldn't come into play very often, but I see a few issues

1. First and foremost I can't see that this would be allowed - this is basically BYU buying designation as a conference champ. At the very least they'd have to have some sort of association with the Sun Belt (probably including a scheduling agreement, which defeats the purpose).

Otherwise, the way you outline it, any team or conference could reach a similar agreement. You could have the SEC reach an agreement that their highest ranked non-playoff team is the declared Sun Belt champ. Instead of AAC Memphis making it in 2019, it would have been #12 Auburn or #13 Alabama. From a Sun Belt perspective, that's better anyway - they're likely to get money more often from the SEC than from BYU and they could strengthen their ties with the biggest player in college football.

2. There may also be years where the Sun Belt Champ is the highest ranked G5 champ and would be in line for that 6th spot, but also be behind BYU. Imagine Appalachian State is #18 and BYU is #16. Then does BYU still become the "Sun Belt Champion" and thereby bump out a Sun Belt team? If not, then it becomes even more legally shady and likely to not be allowed.