You have a lot of options for where you could put a common version of a macro file, assuming you have access to the server side of things.
In localOverrides.conf, you have this line:
https://github.com/openwebwork/webwork2/blob/cfb690ade3d3c55527d522ed1a8f1defd5e19f6a/conf/localOverrides.conf.dist#L267
You can add a path to the list of paths that are searched for macro library files. So (this is just one option) you could make a course that is for development purposes only, and put new macro library files there in its macros folder. Then add that folder to the list of paths with something like:
$pg{directories}{macrosPath} = [@{$pg{directories}{macrosPath}},"/opt/webwork/courses/developmentCourse/templates/macros"];
Then all courses will be able to use whatever is in that macro library folder.