Ok, a proof of concept to have both 15 bit colors with 32 levels of transparency without wasting much space as indicated above. (One could still discuss on how to store them in memory). The zooming is not finished, because I am not sure to continue with it. It would certainly enforce 100% sync, since any transparent pixel cannot be opaque at the same time.
This is still a hack, as there is no compression of identical pixels yet (i.e. a true RLE) and no good zooming, since I am not sure if I should continue.
Thus, seeing how difficult it was to get the desired transparency for a simple tree into GIMP, I highly doubt that there will be many transparency images coming up soon which are not shadow or smoke, and hence blending with one of the 224 nearest colors may do the trick quite well. Moreover, due to the preferred fast calculation (maybe there should be a setting for makeobj to allow on demand only 25/50/75% transparency), there are only 3 bit red and blue and 3 bits green used anyway, thus in total 512 colors. The difference to 224 base colors (about 7.5 bit) is then a 3/4 bit per color and seems minor to me given the double memory footprint and computation effort.