IIRC, the Bx50 would have been an overlay on top of the Bx41, rather than a pure extension.
I would have gone one step further and created a new "borough" designation letter:
- Q70 becomes L70
- M60 becomes L60
- Bx50 (if there is one) would be L50
I'm not sure if a similar "JFK Flyer" branding would be needed (J3, J10, J15), because those routes have huge levels of non-airport riding.