My goal is to have any directory under root that doesn't currently exist to redirect to my file
You can't do something like this in
.htaccess with WordPress (or any site that uses a front-controller pattern) since there is no way to determine what is a "directory that does not exist" and a valid WordPress URL, which also does not exist on the filesystem.
If you implement something like this in
.htaccess then you will essentially override the WordPress site (so none of your WordPress URLs will work).
To implement this with WordPress you will need to do it entirely within WordPress (in PHP). Only when you have determined the request does not map to a valid WordPress URL (ie. would otherwise result in a 404) then you could divert the request to your
redir.php script. (But
.htaccess executes to early in the request to be able to do that.)
Related question on the WordPress stack (closed as being off-topic, probably because it is focusing on the perceived solution (ie.
.htaccess), rather than the actual problem: Getting WordPress to execute a custom function on a request that would otherwise result in a 404):