ZXUno, GBC and the Mister2MEGA65 Framework can be found in sy2002's repositories.
Don't know how official this is 
The development is ongoing and i know, that at least one port is in the make atm. (iirc it's two).
The Devs who are in charge of the porting concentrate on their work, so the Git page might be a bit outdated.
But like you've already found out, they are also working on a MiSTer2MEGA toolchain. Once this is functional
the porting of more cores will become alot easier and faster.
And they are not waiting for the release 
. with the release we hope to get more people onboard to help
on all kind of aspects MEGA65 related. that's the nice thing with an open-source project
There is also a Commodore 64 for MEGA65 project. But also here:
QuoteWARNING: Heavily work-in-progress. Currently this is not even in an alpha state.
This is one of the Core ports that are currently in progress
There is also a Commodore 64 for MEGA65 project. But also here:
QuoteWARNING: Heavily work-in-progress. Currently this is not even in an alpha state.
I just see at this github site from MJoergen that there is also a Commander X16 core in work for the Nexys board (which one that is also in the MEGA65). 
For those who don't know it: The Commander X16 is also a modern 8bit computer.
Wow, that's interesting. I was never too interested in the Commander X16, but having it as a core for the MEGA65 would be pretty nice though
Yes, wow... I was actually going to ask if a X16 core would be possible, but that there is already one in the works is great news!
Edit: hmmm, but it is quit old, last updated 2 years ago...
