[Papa John]

For Harvard graduate students, would it not be possible to enroll in classes at other Boston area schools while performing research and TA duties in person at Harvard?

Not according to current registration/enrollments requirements for the Graduate School of Arts & Sciences. But like you said, Harvard would have to adopt new rules. 

I think transferring credits for continuing students would be especially problematic for the many professional schools at Harvard: Business, Divinity, Design, Education (maybe this one would be easier to transfer credits), Kennedy School of Government, Law, Medicine, Public Health. It might be easier for first-years, but I don't know as I earned my PhD at UCLA and work at a different UC campus, so this is just a WAG!

If this situation extends into the fall, other options for undergrads and non-research graduate/professional students that come to mind are:
(1) leave of absence/gap year until this gets resolved--which it will--and return to Harvard in fall 2026
(2) earn credits at a university in one's home country, if possible given the late date, and return to Harvard, assuming Harvard approves the credit transfers, in fall 2026
(3) leave Harvard permanently for another institution, which would probably require a gap year due to the late date.

Depending on the resolution, a possible fourth option would occur if Harvard experiences a "brain drain" and some international--and domestic-- graduate students follow their former Harvard faculty mentors to other universities.

It's hard for me to imagine this dragging out into the fall, but I never would have imagined this FUBAR conflict in the first place.

[Papa John]

For Harvard graduate students, would it not be possible to enroll in classes at other Boston area schools while performing research and TA duties in person at Harvard?

Not according to current registration/enrollments requirements for the Graduate School of Arts & Sciences. But like you said, Harvard would have to adopt new rules. 

I think transferring credits for continuing students would be especially problematic for the many professional schools at Harvard: Business, Divinity, Design, Education (maybe this one would be easier to transfer credits), Kennedy School of Government, Law, Medicine, Public Health. It might be easier for first-years, but I don't know as I earned my PhD at UCLA and work at a different UC campus, so this is just a WAG!

If this situation extends into the fall, other options for undergrads and non-research graduate/professional students that come to mind are:
(1) leave of absence/gap year until this gets resolved--which it will--and return to Harvard in fall 2026
(2) earn credits at a university in one's home country, if possible given the late date, and return to Harvard, assuming Harvard approves the credit transfers, in fall 2026
(3) leave Harvard permanently for another institution, which would probably require a gap year due to the late date.

Depending on the resolution, a possible fourth option would occur if Harvard experiences a "brain drain" and some international--and domestic-- graduate students follow their former Harvard faculty mentors to other universities.

It's hard for me to imagine this dragging out into the fall, but I never would have imagined this FUBAR conflict in the first place.