I’m not sure! Registries change and modify rules periodically, so depending on the mini’s registration papers and parentage, perhaps?
Hopefully you hear from someone who actually knows!
Many miniature horse “families” have Shetlands in their past so it seems a reasonable possibility anyway! I seem to remember dual registered small equines in the past.
Shetland ponies are registered bloodstock, if they don't come from registered ponies, they can't be registered as Shetland ponies. Under the ASPC/AMHR registration there is a division for ponies that have just one registered parent (this division may be a place where those over sized minis can go, but they can not go into regular Shetland registry).
Right, shetland is a breed, like AQHA or Arabian. Currently you need 2 registered Shetland parents to get a shetland pony. Put a shetland pony that's small enough can be registered as a miniature horse. That's a height registry like Pinto is a color registry. As long as you meet the requirements anyone can get in.