My two leagues have multiple sanctions but use a different rating system. We pay sanction fees to multiple organizations so if league members play tournaments they can play. About 80%-90% of our league players play tournaments. Our league members that want to start playing tournaments are given a starting fargo by our league operators as with so many league members already shooting tournaments it is easy for them to pretty accurately judge where they should start based on their league play.
They will usually assign them a starting fargo slightly higher than what he thinks it should be to start with. When my wife first wanted to start shooting tmts, he gave her a starting fargo about 30 points higher than she actually is after about 200 games in fargo. These league operators have been doing this for years and years and are well respected by the operators of all the tournaments in our state.
Edit, If you have never played league under BCA sanction, not sure what you can do as we routinely have people looking to sub when it close to big state tmts. I think they have to have 2-4 plays in in a league that is under those sanctions first.