Is there still a market for such things ?
Well yes, but I think if you're going to be hard-nosed and commercial about it, you've got to recognise that it's a much smaller market than in the past. How much smaller? The closest thing we have to actual data is the Great Wargames Survey from WSS, and that shows:
"We asked respondents to indicate preference in order, so each size or scale got an average score. No prizes for correctly guessing number one: it’s 28mm with 7.75. 28mm Heroic, listed separately to accommodate certain well-known miniatures, got 7.11, very closely followed by 15mm with 7.04. There’s then a small gap before we get to the next three, very close together: 20mm/72nd scale with 5.94, 6mm with 5.88 and 10-12mm with 5.58."So it looks like 20mm is about the size of the 6mm market. I understand that data is across all periods, I wonder if you could get in touch with Jasper and get the data broken down just for the period in question?
Having said that some traders seem to do ok providing niche conversion kits in 20mm, such as S & S models, but they also do other scales.