I like the idea, especially for removing the charge at the end of the day, it might work there are some concerns...
IMHO,
What would you use as a blank? a 45-70 case gives about 70gr (volume) of course you could go 45-90, 50-70, 50-90 50-110 or something trimmed between.
To avoid detonation, the bullet needs to be seated atop the powder without an airspace, thus some sort of wadding would be required to keep it all "solid". This may limit options for reduced power loads.
Finally, if cartridge of similar bore size could be seated in the breech, you'll have a hard time convincing the AHJ (authority having jurisdiction) that the rifle isn't a breech loader.
If you don't have the dexterity to load a 209 primer you could try one of the 25acp conversions.
Just my $0.02
Jim