Does anyone know if it's possible to have Lotus Notes & Domino do external authentication via RADIUS? Specifically to integrate a 2FA solution.
No, unless RADIUS can use IBM's Lightweight Third-Party Application (LTPA) tokens.
Here's are two workarounds. I should caveat that most them will only work for web applications (Domino and XPages), native Notes client (at least in the case of R6) support is limited to single sign on (by sync Notes ID password with AD) or SmartCards.
LDAP - If RADIUS can expose it's data as LDAP you can use Lotus Domino's Directory Assistance to set this up as a 3rd party directory.
AD as a bridge - If RADIUS can communicate with AD, then you can use AD to authenticate Domino sign ons.