I don't know if it has been mentioned here before, but I seem to have found a way to plan which league sessions I want to have a go at.
The scheduled sessions found on the website, use the same query as the one used in Bsim - Leagues. So the first 15 sessions found on the website is the ones you will see in the BSim Leagues.
So, the trick is to manipulate the list found at the website, since the filter for Leagues in BSim is useless.
You can do this by going to the list of active tages and just click "copy". This will make a session that inserts itself somewhere in the first 15 session. It does not seem to be 100% consistent, and sometimes more than one copy will be inserted, but it's still better than the BSim League filter.