I've seen a few achieve it. One way I've seen, not sure how well it works, is to place newspaper everywhere during floor time and then put it in the litter tray and hope they associate with the toilet. You could also try just seeing if a piggy goes in a specific corner or place and put the litter tray there. Perhaps you could place newspaper where a piggy has a specific spot to go and in a while put it in the tray to attempt to associate it with that. I generally give up on litter training lol as most, if not all, of the piggies I've had just go everywhere
Edit- my boy Ginny just now made sure he went on the puppy pad before walking off (however it was folded about to go in the trash and so the floor was instead used, but he tried aha)