Hi there,
The best way to control Session visibility would be by Registration Type as you mentioned. You also could consider breaking out your Admission Items further, as you can control which Sessions are visible within the Advanced Settings tab of the Admission Item, as well.
If these options do not work for you, you do have the option to bulk-update Session registration on the back-end, as long as the Session does not have fees. To easily sort through your list of invitees, you can import an Internal Information field with the Session name, and then apply an advanced search within Invitees & Registrants. Then, under the Bulk Actions dropdown you will see the option to Update Session Registrants.
Best,
Danni