Say you could build a 50 kg A cat with 25 kg of movable weight added which can be displaced to windwards with a sheet. Would this fit the current A class rules?

It all boils down to the question if the A class really wants to be a development class or if at some point it just wants to abandon certain developments, which requires more and more restrictive class rules. The foil/wing is a secondary, although actual, issue, imho. More issues to come, for shure.