I think you might benefit from checking with ASSRA folks. They have a lot of experience with the cartridge. Some still use it in competition.
http://www.assra.com/cgi-bin/yabb/YaBB.pl

Hodgon has some data, but not for a bullet that light:
http://data.hodgdon.com/cartridge_load.asp

This is the loading that Steves Pages show:
http://www.stevespages.com/321_2_170.html

I think you would be better off dropping down your charge weight rather than bullet weight. Some people report having problems casting even the heavier 32 bullets. If you are going to use smokeless, look for a slower burning powder.

I have spoken to Hodgon tech support about a similar situation, the 44-40. They saw no problem with loading only 15 grains of Pyrodex instead of the normal 40 grains. None of their data suggested a potential pressure issue with doing that.

Pete

http://www.damascus-barrels.com/Movie.html