Hi Russell,
This is already possible today using permissions. Sharing Termbases was an initial mechanism we had a few years back to share with users outside a tenant, before we had more comprehensive permissions (we are still debating if we should retire this mechanism eventually).
You essentially can grant access to Termbases by placing them in a Customer and then granting permissions (either read-only or read-write).
With SSO and Automated user provisioning, you could also automate this process and grant certain users access to only those termbases when they first login.
Thanks,
Luis