We do know that Rumple’s been there before or has some connection to it- Adam and Eddy stated as much. But I suppose Regina *could* have been before- perhaps through Jefferson’s hat. I guess we will have to see.
With Regina’s mention of mermaids, and mermaids being in NL, that’s sorta the go to connection, though I dunno about that, because if there’s some annoying mermaid in NL, why doesn’t Regina just stay put in FTL so she doesn’t have to deal with the mermaid? But instead we see her going to Rumple for help.
If Regina’s been to NL before, I think it may have something to do with acquiring the failsafe diamond. It was something she seemed to have included in the Curse of her own doing, but its purpose was to basically destroy everyone else and isolate Henry, and isolating Henry is what PP wanted, so I think the failsafe has to connect back to him somehow, which means Regina got it from NL somehow.