Hi,
I've never been in a U21 crew (nor NT), but one thing i'm sure of concerning (some) small countries : it happens that there are very few "dedicated" managers. It makes it harder to make sure that the NT players get their GS and training etc. etc.
The idea about getting first some experience in assisting/scouting for a NT/U21 as a staff member sounds the best IMO.
Good luck dude ! ;°)