|I've already cut/yanked the root out of the line, and everything is
|draining great. Now it's just a matter of figuring out how to patch that
|cast iron joint--it's a collar type join and a big chunk is torn off.

Professional plumbers have things made for just such situations.
I would consider calling around. You have already done most of the work, so the
actual repair should be relatively inexpensive.

Alternately, find a plumbing supply house, take photos of your problem, and ask
what they have to address the problem.
Rex in Fort Worth