I think the idea bears merit, and we might need to view this through a slightly different lense. Many of the commenters on this forum fall squarely into the deep end of the enthusiast category and have suggested some very interesting, but not readily available options for chambering. I don't necessarily think the target market here is for someone as well versed in the double market to buy a used Chapuis, Heym, etc., but rather someone looking for their first double for a first DG safari. Guided hunts in the US have skyrocketed in cost to a degree where an African safari has become something of a budget play (obviously there are degrees of cost here). Certainly there's been a definite surge in interest over the past few years and the outfitters I've talked to are seeing strong growth in first time safari clients.
I certainly see the appeal for the .45-70 for Alaska, and the .375 H&H would seem to be almost table stakes for an African chambering. If we look at ammo availability, I always see .375 and .416 ruger in stock and there's plenty of availability on Ammoseek for both. Hornady has done a great job in reviving production for some African calibers and seems to be developing new ones to fit the mold, might make sense to pursue a partnership with them. Given what I think the target market will be here, the end user is likely not someone who will have a reloading setup for some relatively obscure African round and will just value ready availability in their regulated load.
Envisioning the experience of a first time buyer here, you could make an add-on package (again, partnership with Hornady could make a ton of sense) where the customer can purchase 200 rounds of the regulated load and an afternoon of instruction. Thinking of the clientele and the physical points of sale for Chapuis near me, this is something I absolutely see selling well at the Beretta Gallery in Dallas, potentially with a partnership with Dallas Gun Club or ETTS where you could set up a bay with buffalo targets or something and help the client get used to shooting off sticks and such.