Direct port with just external tweaks is impossible.
But i'm sure with some deep modification of ram handling inside the core the port is possible.
MiST and its clones are in the same boat and will require the same in-dept changes to port the Next, so i suggest to widen your search for someone to port. If someone will port it to MiST, then it will be easy to port to MiSTer from there. Ask Gyurco (Slingshot) if he is willing to port to MiST
I know he is a fan of ZX Spectrum.
As i've told earlier, i'm not really interesting in Next so i don't want to spend my time for core which is basically the same as existing ZX Spectum core. So i'm not willing to do in-dept modifications to port.
The biggest issue (if it will be ported) with Next will be requirement of VHD image. So you will have to put files inside that image to access them in Next core. It's very inconvenient. My ZX Spectrum core works with files on SD card, so it's a way more convenient to manage.