The Republicans would be foolish to boycott a joint session of Congress with the President in attendance. Unexpected laws could be passed by acclamation and signed into law before the Republicans could protest. Or six new Supreme Court Justices could be appointed by the quorum present if regular rules are suspended due to the absence of the GOP.
Seriously, it’s a bad idea.
A Joint Session is ceremonial and cannot pass laws, although they can count Electoral Votes.