If they were to put the effort into creating a league or tournament, they could make a rule that only players drafted by the team could play. They already have rules about timing of purchase for BBB/M, cup and playoffs that wouldn't be hard to extend to disallow purchased players from selection. So technically even non homegrown teams could compete if they had 5 eligible players which I think would still be in spirit of the idea.
This is one of the reasons why it's such a great idea to have a discussion - I had not even considered that. It could be a very interesting possibility if all teams could participate, but with their homegrown roster only (if that would be easy enough to implement code wise). Should BBHG league allow these kind of teams to participate... after just seeing this idea I personally wouldn't be able to make up my mind, but nevertheless it's an interesting take on it! There is a chance that it could make becoming homegrown and training your own players more popular if many teams would get to participate in such a competition in where they'd get to see the effects of their training against the opposition.
Whether it is worth it to create such a league is another question entirely. There is a very small fraction of users that are homegrown or even train their own drafted players. And training your own players sort of goes against the current U21 system which is very popular. So they'd conceivably be at odds.
True - however, whether or not something's worth of something is always up for discussion anyway. For me personally creation of such league would be the best thing that ever has happened to Buzzerbeater.
Plus, especially if league would be official, I think it would encourage more managers to become homegrown and make homegrown teams more tightly knit community so to speak. I mean for us, it's often fighting against the windmills as we are competing partly against the money (and not necessarily against the skill to train and play your players optimally) and it would be a well needed breath of fresh air to be able to compete in a level playing field agains fellow HG managers.
Your point about training to U21 teams is another interesting discussion, however, it shouldn't be the reason to not start this kind of homegrown league.
I find it somewhat contradictory how players are trained for U21 (vs. how they are usually recommended to be trained if you aim to train them for national team, or if you plan to train a player for your own teams' needs), as I don't think that would resemble well the real life - in real life you'd probably train the promising player relatively the same manner whether you'd train him for U21 or for NT.
Anyway, like you mentioned, some managers would find training with the "U21 training restrictions" in mind, some with "national team training restrictions" and some would just train players for themselves (and, please bear in mind that most still perfectly trainable/potential players are not good enough for U21 or NT anyway).
Personally I've done all of those types of training (U21, NT and for my team) and I don't see it contradicting with the idea of homegrown league in a major way - in order to participate, there's no requirement for you to train all or any of your players optimally (or even train them at all).
Another idea would be to implement ladders for specific things. Season of team creation, under 21 players only, homegrown, and let people use scrimmages to advance up these ladders. But that's a whole lot of effort for maybe marginal return. I think the owners are very focused on UI and code optimization right now.
That would be interesting possibility and in optimal world all of those would be available and all would decide to participate.
But more realistically I'd prefer a larger amount (than 3 currently!) of participating teams, so I'd say it'd be better to focus on promoting this BBHG league. :)