This link shows how to do it, How to Configure UAG to Publish Your Private Certificate Revocation List - The Cloud Security Man - ...follow the steps to use UAG as a simple reverse proxy to your webadaptor link. Just replace references to CRL to your webadaptor name. For example in step two
put the trunk name in as something like ArcGISMaps and the public host name as the name you used in webadaptor for example webmaps.yourdomain.com
In step 5 add the address as the fully qualified server name that you have webadaptor installed on and put the path in as the name you used in webadaptor (default is /arcgis)
Follow the sets in the link and at the end Select Trunk Configuration and choose Configure. Edit the custom rule under URL set that was created and switch it from Ignore to Handle
Select ok, save and activate UAG. This worked for us.