@jon If it happens I'd - oddly enough - expect it to be from ÖBB rather than SŽ or HŽPP, since they currently provide the loco on the Slovenian and Austrian parts of the ECs and they could shuffle carriages/trainsets between countries more easily than the Slovenians or Croatians. The Mimara already used to use ÖBB coaches entirely when it went all the way to Frankfurt so it's not all that unlikely
I'm sceptical of the split of the Pula train being for economical reasons - presumably the current train is already operated jointly by SŽ and HŽPP so splitting it wouldn't save much money; more likely that this is somehow related to the new Koncar battery units that are about to enter service (battery chargers are already being installed). Perhaps they wanna completely de-dieselify that route for passenger ops and of course the battery trains aren't certified for Slovenia, necessitating the split