Ignoring whether this would actually happen, is there any rule that would prohibit a NFL team or unit (OL, DL, etc.) from practising with a NCAA football team or unit?
Would the NCAA consider it tampering in some form?
Yes. Not even getting into the weird NCAA rules - which I am sure this would violate many - it violates the current collective bargaining agreement in the NFL.
First NFL teams can only practice with other NFL teams in designated scrimmages before the start of the regular season. And simply any player practicing with a team (or a team of players) without an NFL contract would be in direct violation of the NFL player's union contract. Coach's can't even give a former NFL player a try-out during a practice. No contract = no practice.