Your dedication to producing these rimfire cases is admirable!
I fully understand your desire to fire it this way, but have been wondering if it is possible to alter one of these carbines (e.g. with a separate modified block or the like) to use centerfire cases?
I understand Spencers can be modified to centerfire ... and I have a Canadian-contract Peabody rifle chambered for .50-60 Peabody rimfire, for which I was able to get a centerfire block made from one of the "new, old stock" Peabody breechblocks that Dixie Gun Works used to have (...original breechblock was left unmodified, of course ...)
My original .50-60 Peabody is on the top in the attached composite and I also I have a modern reproduction Spencer rifle which is chambered in .56.50 ... but of course it is centerfire, so I use the Starline cases I have "as is" ...
Canada acquired quite a few .56-.50 Spencer Model 1865 rifles and carbines at the same time they acquired 3,000 .50-60 Peabody rifles and, because the Peabody could chamber and fire the Spencer cartridge, but the Spencer would not cycle the Peabody cartridge, the Militia Department acquired only .56-50 spencer cartridges for use in both, to avoid the logistical nightmare which otherwise might have ensued. Something I have always meant to do, but have not yet gotten around to, is to fire my Peabody with Spencer cartridges and see how they perform compared to the somewhat longer .50-60 Peabody cartridge (for which I use shortened .50-70 brass.)