Logging actual fetched URL after request is re-written

[Franks Andy (IT Technical Architecture Manager)]

Hi all,
  Logging with HTTP as standard, the %{+Q}r log variable records the requested 
URL in the logs. I'd like to also record the URL that's actually fetch after an 
http-request set-path directive is given (for debugging purposes). It's linked 
to an application that provides next to no debugging, and tcpdump isn't much 
help either - having it in haproxy logs would be really useful.
Can I do this or am I thinking too much outside the box? I tried setting a 
dynamic variable and then setting that in the frontend log-format, but it 
didn't seem to record anything even despite populating the variable.
Thanks