This will require us to leave login credentials on the server that is pulling the file, no? I was more interested in some limiting the scope of access to a single project -- thereby something like <some_access_key>.

@foven we have not found a solution. We had run out of time to debug SVN. Now we only host GIT repos on Phabricator and SVN is elsewhere.

There is a default pre-commit hook. I don't any custom ones there. If I run it as vcs-user I get a error at this line:
throw new Exception(pht('usage: commit-hook <callsign> <repo> <txn>'));
I'm hesitant to call this exception the same as the issue I see during actual commit because there is no obvious link.

Still looking... nothing useful found. Anyone know what information might be helpful to debug?