I think I know the answer, but wanted to confirm.

With a 458 AR, or 458 B&M, or 460 Short A-Square, how many can you fit in the same mag space as 4 standard mag belted cartridges (458 WM belted derived cases)?

I believe 3 of the AR/B&M/S-Square will fit in the same mag?

3, and with a little work, 4.
the B&M/AccRels use RUM cases as basic.. .550 nominal base diameter.

the a2 uses belted weatherby cases, right? .602 ish belt diameter ..

can have more .550 than .602

Are there any additional cartridges (in terms of the same length and power capacity) I should be looking into?

Here is one that "Might" fit the bill.
Its not a wildcat but its close.
12.04 Lazzeroni Lilmufu
Its a 475 Short Mag that Lazzeroni says pushes a 500 grain bullet at 2700. I think that is a little bit of a stretch, 2500 I would believe.
I dont know what the exact case measurements are but I think it will fit.
Factory cases with the proper head stamp are $49.95 for a box of 20.
That is a good price.

Good luck with the project, I'm looking forward to reading about it.

Cheers, John

the enfield action can do a 600 overkill .. 550 magnum, 550 express, 550 gibbs, and all the 460 weatherby based cases.. you can fit whatever you want, really...

i, of course, prefer the accrels!

but, if you want the best-s, that the 470 mbogo