Richard, for MacVim, there is no MacVim specific way to make different tab have different fonts. But right now you could potentially set up autocmd for TabEnter to change guifont to whatever you like.
Regarding goneovim, I tried that feature out. It seems to still be experimental according to the README? It seems to be quite buggy as the moment I tried resizing a vertical split it starts to not draw correctly or leaves blank space. It's essentially permanently detaching said window using multigrid and so doesn't respond to split resizing anymore. Maybe it's still WIP? Still useful though.