A few years ago John Sykes of Hydrographics fame used to regularly shoot NEFTA winter league from his wheelchair. He used a cushion to rest on the arm of his chair for sitters, one elbow for kneelers and free sitting for standing. I cannot remember anyone complaining that John had an advantage. The current Health and Safety brigade would have kittens if they saw what we used to drag him round.
The problem I see with a separate sitting class is the trophy hunters who would spoil it for the genuine physically challenged, but you get them in all forms of sport. If you are going to try and find a compromise why not introduce it at regional level for next years winter league and ask the regions to monitor it. This would give all interested parties a chance to see it in practice. The rules could then be tweaked for future years. For the inter regionalís you could limit the teams to one person only from the sitting class, provided he or she made the top 15. Once satisfied with the winter league rules they could be introduced at national level.
NEFTA run a recoiling class, which had 8 shooters this year, with only 2 shooting regularly, so low numbers would not be a problem. Trophies for the sitting class could be awarded subject to entry.