I almost exclusively use web-routes behind a proxy, so I am not quite sure if I understand your issue.
web-routes generates relative urls which are appended to whatever baseURL you provide. For example if you have something like this:
implSite (pack "http://localhost:8000
") (pack "/route") site
The first argument is where the 'absolute uri' part is coming from. If you changed that to (pack "") then it would just spit out the relative URL.
Alternatively, you could provide the external address that users should be visiting rather than the internal name/port.
Does that make sense?