I suspect the obstacle to something like this would be that it complicates the performance profile of anything that needs to render the dropdown (_admin, Asset Builder, etc). At the moment it’s a straightforward “Just load the Select Metadata Field asset and its attributes and you have everything you need”. If it could be dynamic, then potentially it could take (much) longer, require (many) more queries, and could fail in various ways that you don’t need to worry about if you’re just loading a single attribute.
Best compromise might be a process to assist with updating the options in the Select Field asset - the options are stored in a serialised attribute called
select_options that can be set by one of the APIs (JS or SOAP). You could make a utility page that grabbed the listing, PHP serialised it and then called setAttribute to update it.
It might even be possible to fully automate it with a trigger … but I wouldn’t.