I guess what I want is a tool (not necessarily mr) that I point to a GitHub/Bitbucket/Gitlab project and it creates/updates the .mrconfig with all repos from the project.

I guess I would generate mr config commands using the output of gh/glab repo list and run them in shell:

gh repo list --json url,name -q '.[] | "mr --config .mrconfig config " + .name + " " + .url' | sh

You could probably do it with xargs or similar too.