Howdy,
The US Govt actually issued a simular load for the old trapdoors, but they were loaded with smaller shot (00 I think). If you can find it, there is a magazine article printed in the National Rifleman entitled " The 45/70: from mice to elephants" it lists several special purpose loads for the 45/70. You might also have your friend go to evilbay and buy one of those "complete" load books for the 45/70.
Since you are talkin' about .457dia bullets, I wouldn't think pressure would be too much problem since they are well under bore size for a .458 bore, But if I were doin this, I would try either fffg black powder or cleanshot or, H777 until a knew for sure the bullets were gonna act. My favorite plinkin load for my single action 45LC's is a .457dia swadged round ball over about 30gr of fffg 777with the all seated about halfway in and lightly crimped.
JMHO,
DT