Cisco Application :: ACE 2.2 URI Rewriting

We've got an application that broke after upgrading our ACEs from A5(2.1) to A5(2.2); the problem lies in how the ACE handles URLs with embedded backslash characters in them - e.g.: URL
 
Prior to the upgrade the ACE would forward these to the back-end servers; after the upgrade the ACE resets the client connection.
(We're doing SSL offload on the ACE; the back-end connection is HTTP over port 80, only the client-side traffic is over SSL.)
 
Some browsers will convert these to percent-encoded form - i.e. URL
 
and things work for these; but other browsers won't do this.  So I'd like to set up a rewrite rule in the ACE that will replace any (or at least the first) '' with the string '%5C'.  Just how to do this isn't clear from the command ref, and the config guide is a tad shy on similar examples.