Listing all SVN repositories

No. Each subversion repository is independent and knows nothing about the others. There would be no way to locate all of the subversion repositories.

We keep all of our repositories in a standard location (/data/svn) and can easily and programatically list all of the repositories:

ls /data/svn

Or from a remote system:

ssh svn ls /data/svn

SVN repos have a distinct structure (/conf, /hooks, /db, /locks), so searching for one if its components would be a start.

If locate/updatedb is installed and current:

locate /db | grep /db$

Or by an exhaustive search:

find / -type d | grep /db$

A more sophisticated search would check for the full set of components.