HOOK Script Question: how can I determine url from a transaction with SVNlook

How can I add custom text to the error output for a commit?

I run a repository with http access for browsing and svnserve access for
commits. I keep on having users checkout with http, not be able to check
in, and then I direct them to my "svn switch --relocate" doc. I'd like to
have the svn failure message for the commit return text that directs the
users to that doc (so I don't have to). I already do this for other
situations (like no checkin comment) via hook scripts. So, I'd like to add
this one to the hook script as well.

My problem is that in the hook script I can get at the local path to the
repository and the transaction name.
What I'd like is the url that was used.

Is there any way for me to get at that or am I trying to reach my goal via
the wrong path?

-- 
Peter Kahn
citizenkahn@gmail.com
citizenkahn@jabber80.com, pkahnpie1@AIM, skype: citizenkahn
http://analogoustendencies.blogspot.com/
Awareness - Intention - Action