Today it occurred to me that numeric arguments 0 and 1 could be utilized for different purposes. For most commands they are meaningless which allows us to assign them a specific meaning ourselves. Here is one way, which uses numeric prefix 0 to projectize commands:

Using this you can call C-0 C-c r or C-0 C-c f and will be prompted with the (git) project root (if it exists). This also works when you use C-0 M-x rgrep to call rgrep. The hack above should work for many other commands which prompt you for a directory.