Don't fight about it on my behalf, guys. I appreciate all the helpful tips, even if some needed clarification.
I struck out on finding the desired repair ring at Lowe's, as well. That's always my first choice, but Ace is much closer. Rather than drive around town trying Home Depot, George Morlan, etc., I went with one of the partial repair rings. It's about a third of the full ring, designed for patching broken pieces of the flange. I cut off the bad bolt and secured the partial ring through the lead and through the mortar into the sub-floor. I couldn't follow the advice of a double nut to hold the new bolt in place, as there wasn't enough clearance for it under the toilet.
And yeah, the wax ring fits entirely inside the bolts, give or take a fraction of an inch, so no extra sealing is needed.
All in all, I think this will work, but it's definitely jimmy-rigged. Next time I have to do a toilet replacement (hopefully a long time down the road!), it'll probably require hiring a plumber to cut the old flange out and re-do it properly.