Subtract the volume of the plug, add what you get back from no brass case, add in the taper and freebore limiting how far back you could seat the bullet, and you'd probably come up with about the same chamber volume the .458 started with,
Normal engineering standards say a threaded length of 1.5 diameters are are strong as threads get, so a breech plug from 5/8 to 3/4" long would do.
Black powder performs a bit differently than most smokeless. With black, there's usually no real advantage to a necked-down case, so they were either straight or had long sloping shoulders to neck them down to smaller bullets.
Smokeless powders typically perform well in necked-down cartridges, and even the angle of the neck makes a detectable difference in performance. PO Ackley used to play with that kind of stuff.
Anyway, there's no reason you can't plug the chamber and run full .458 smokeless loads. It would be interesting to see how an AK receiver would handle that sort of thing, though.
Of course, being an AK, it's trivial to re-rivet or replace the receiver if necessary...
I was looking at building a black powder rifle a few months ago. Barrels are relatively inexpensive, but the lock mechanisms are outrageously priced. So I started looking at inline ignition using ordinary rifle bits, then realized there was no reason not to run smokeless if I used an appropriate barrel, and then found out Savage was already selling smokeless muzzle loaders, and... it was another "learning experience."