I don't know if it would suit what you want, but Kogswell was doing some 650C frames. Might be nice for you anyway!
Lug wise, I haven't built a 650c, but I don't seen why you couldn't use standard lugs. Where that can get iffy is in making same size frames, but with a different size wheelset. But if you are scaling a frame down, that otherwise could use a set of lugs in the larger size wheel, then I don't think you have a problem. One is scaling down all the parts. On the other hand, trying to make a lugged touring bike with frame lugs designed for 700c with 26" wheels can get interesting. To maintain the same BB height, with smaller wheels, the BB lug angles flatten out.