My brother used to shoot light (100gr. ) loads in his .45 3-1/4" Sharps and achieved excellent accuracy using card wads to fill the space between the bullet and black powder. Black powder likes a bit of compression - maybe up to 1/8" in that long a case.
I don't see why the rice wouln't work, as long as you've a wad between the rice and powder and another wad between the rice and the bullet. Rice is pretty heavy.
The long stack of wads my bro used with his 2F loads didn't raise pressures that were recordable, but shows that this is quite relaxed, non critical loading. Just about whatever works is OK.

I would be inclined to use a wad over the powder, compress that 1/8" using and plunger (wad seater/compressor) in a loading die mounted in the press. Then fill the case to the top or almost to the top with something easily compresssible, like Winchester's Grex shotshell filler, then seat the bullet down to whatever level you need for chambering, ensuring the Grex is compressed as well.

Merely keep in mind BP likes some compression to burn well.

You might find best accuracy in that long 3-1/4" case if using a magnum primer. BP is easily ignited and doesn't need the extra flame merely for ignition, BUT, seems to be more accurate in many rifles if one is used. Even my 1.9" (.45/60) Sharps shoots better with a magnum primer than with a simple large rifle primer.