In my experience, this works just fine, despite the glaring disclaimer (which doesn't stop me, because I have elected to ignore it).
Extensions just need to do the following:

• Create a directory to store CSS and JS in. Phorge uses rsrc/ within webroot/, which is misleading, because it does not have to be a web-accessible directory.
• Create a directory where Celerity should store its map. If you already have a resources/ directory, you can create a celerity directory here.
• Create a new class extending CelerityResourcesOnDisk and link these paths. CelerityPhabricatorResources is a good example.
  • If you do not have a need for packages, you can just create a packages.php file that returns an empty array.
• Create some resources, following the documentation on how to do so.
• Run bin/celerity map, which will rebuild for all Celerity resources.
• Access the new resource by using require_celerity_resource('NAME_OF_RESOURCE', 'EXTENSION_NAME').