Nebraska is not an AAU member. They were on the verge of being kicked out of the AAU when they were seeking to join the Big 10 in 2010. It was known that their AAU status was in jeopardy while they were seeking to join the Big 10. They were voted out of the AAU in April 2011, several months prior to their membership in the Big 10 becoming effective. If AAU membership was a dead set requirement, Nebraska would not have been voted in while their status was in peril.
That aside, all the old paradigms are dying anyway. Nothing that has been in college football matters going forward. In fact, there seems to be a concerted effort to abolish those things that have been. The singular determining factor in every decision going forward is money. Regarding future expansion, at the most, AAU membership would be a preference for Big 10 membership. Regarding future expansion, if the Big 10 could plant a flag in SEC territory, it'll be AA-who? instead of AAU.