Indeed thats a great idea. The downturn I see in int? approach:
1- (annoyance) The helper functions will return int? instead of the EntityType enum. This meanes consumers have to always check for null and then do the appropiate casting
2- (issue) some developers still tend to define a variable as EntityType and do not initialize it. This tends to introduce runt-time logical errors that could hard or tricky to catch specially in projects with multiple developer teams.
That said I do agree the int? is a viable option though having Unkown=0 as the first enum value is safer and more conveniant.