Thanks for all the feedback.
Since it seemed clear no bottom bracket post was going to just fall into my lap, I got a bit more creative. Some leftover blocks from a tubing block building exercise, a bit of shimming, and a "custom made" spacing checker served to give me what I needed to bring the rear end to 120mm spacing and much closer to proper centering with respect to the seat tube.
That allows this project to continue moving forward. A better long-term solution will have to wait until after the new year...