Great idea! I had to tweak the regex to -/#[a-z]\w+/, though, since notes referring to the #1 thing you need to do will otherwise get falsely considered to have tags. 
Another weird trick you can do: if you start all your tag names with /, then searching for -tag:# will actually work. (It’d be nice if Obsidian’s search just took -tag: or -tag:# to mean “find untagged”, though.)