Looking at almost all double ring chainsets, the inner ring is usually no more that 30t. However, if you wanted to convert an existing triple chainset to a double, you couldn't go any smaller than a 32t inner ring (due to bcd/tooth number issues).
So........
What about if you used the granny ring tabs to mount the inner ring, and then you could have 30t (or less) chaingrings, without any bcd/tooth number issues. You could then mount the outer ring where previously the middle ring was mounted (and use chainring bolt spacers to take up the slack).
The only potential issue which springs to mind, could be the front mech clearing a larger than normal granny ring?
