Tatume - I would try a Lyman mould in HP - maybe a pistol bullet and paper patch them to .457" - if the HP plug was enlarged and lengthened, causing the hole to run further in, at about 2,500fps or so, they should completely break up. Of course, this is speculative and would need a lot of testing.

High antimony content, as in Linotype, would be imperative to get the brittle nature needed.

The HighWall will of course, increase speeds over what one might expect in a lever gun.

I think RCBS still makes a DC with gas check in 300 or 320gr. weight. Years ago, I had one for my .45Colt M94. As it was rifle calibre, I had to lube, then size them down for the .45Colt's smaller bore and groove diameter.
I hollow-pointed the front cavity and thus cast a 320gr. flat nose gas check bullet as well as a HP every shot.
I used the HP bullets case in WW for elk, shooting 2 with it - worked well at 1,584fps, which duplicated the .45/90 BP load by Winchester.
That RCBS mould, if HP's might be a good one for the High Wall running 2,400fps or maybe more. The size of the HP hole could be large enough to get them down to 280gr., I suspect.