You'd probably be OK as long as the alpha extremes (0 and 255) were lossless; everything in between could probably cope with being slightly off, especially if you could control the quality of alpha and YUV separately, and the YUV quality was (somewhat) weighted by alpha, with a 1 pixel "border" into transparent areas to support interpolation when scaling. Lossless alpha and lossy YUV is already a major improvement over the status quo, however.