Btw, thank you Tomás, for your generous donation, it helps paying forward development!
This feature is now implemented in just released nighty, but slightly differently, following discussion on its tracker:
forge.joomlapolis.com/issues/8878
Subscription plans got a new setting, in "Workflows" tab, "CB Login workflows" fieldset: "Allow login" (default ON).
This "Allow login" setting is available only if `enableFreeRegisteredUser` is not ON at general config,
For plans that shouldn't allow login, just switch this to OFF.
Advantage: No mixup with Exclusive, or different Exclusive settings, and maximum flexibility. Backwards compatibilty (default is On).
Disadvantage: CBSubs plans displayed when a user hasn't at least one active plan that allows login display as before. It's up to the administrator to make it clear to the user which plans are required to login (e.g. by making them exclusive at top level, or making it clear in description).
Please check if it works for you and let us know here.