What would it take to have "vim-wiki" for nano (see also: FOAM, Obsidian)? I have always admired nano for how simple and easy to use it is, and would love to use it for wiki/obsidian-style note taking & linking...

Nano doesn't really support any sort of scripting or plugins beyond macro sequences AFAIK. So, not really in the same sense as something like vim-wiki etc. You might be able to cobble together something with shell scripts and text files, but it won't be well integrated.