My guess is that most custom frame builders will do S&S couplers. As long as the tubes are round, they can put the couplers on (e.g., one of Waterford's steel models has oval tubes).

I love my S&S coupled bike so much we bought one for DH. Both are Waterford RS14s. The couplers plus the suitcase run around $1K so a complete bike for $3K I suppose is possible. Like Knotted said, there is the Surly Travelers Check for an off-the-shelf option.

If you go that way and you need packing suggestions, I believe that DebW posted packing pictures and I can send you my cheat sheet. We don't dissassemble as much as DebW, but we do pack both of the bikes the same way even though DH's is sssooooo tiny compared to mine. For us, it is just easier to do them the same.

Good luck on the hunt.