I believe the manual injection doesn't apply in this case, because the responsible for the manual injection should not put the RUM code in the URLs he wants excluded.
Regarding the browser extension, I don't have practical knowledge there, but it seems that using the URL injection pattern might not be useful, as it seems to lack a negating function...
You're quite right @AntonioSousa. Unfortunately we came across a customer that deployed the tag with a tag manager that can only deploy the tag for the whole web application, and not for specific sections. I'll have to see if they can apply exceptions to ignore. It would be nice if we could filter, just as the OP suggested.
The alternative is not using the tag manager and have them apply truly manually the token, but this is quite cumbersome for a large site, and changes will often need to go through static release windows.
The browser extension delivers similar result, but by using different patterns you should be able to split up a site in sub sections (site.com/section-a, site.com/section-b)