Re: What powder for 45-70 w/16" barrel?
Same powder that's best in a 20, 24 or 30 inch barrel.
Barrel length has no influence on the powder that generates the most velocity, if that's what you mean.
Fillers, if you are able to use them correctly, are helpful with loads that are low pressure and leave a lot of airspace in the case, excepting to a considerable degree the fast pistol/shotgun powders which tend to be less position sensitive than rifle powders used at low pressures in this straight case. That's why Swany mentions that Unique doesn't need filler.
Corn meal is not my preference for filler, and full power loads don't need it.
I would not combine corn meal filler with fast powders like Unique so again I second his comment. The reasons why are many. Bottom line is it ain't a very good idea. I can elaborate should you wish.