Maybe you could use the following rule:
Odd Years (NT qualification, U21 WC)
U21 has priority for teams that remain in U21 WC.
If an U21 team does not qualify for the U21WC or repechage, the NT has priority for the full season. Once an U21 team is eliminated (after the repechage, 1st round, or 2nd round) the NT has priority for the remainder of the season.
Even Years (NT WC, U21 qualification)
NT has priority for teams that remain in WC.
If an NT team does not qualify for the WC or repechage, it may not use underaged players. Once an NT is eliminated (after the repechage, 1st round, or 2nd round), they can not use underaged players. If there are underaged players on the NT, the U21 may add them to their roster.
If a player is moved to the team with priority, there will be no enthusiasm loss for the team he is removed from. So the basic principle is that underaged players may be used by the NT as long as they are helping them win a competition, but the NT may not be used as a taxi squad for the U21.