First option we looked into was MapProxy . It's relatively easy to install and to get a basic sample up and running. But it seems integration into IIS is not straightforward. I didn't bother to invest more time investigating this option any further. Fortunately, GeoServer also supports reprojecting WMS services - and surprisingly it was very easy to install and to get it to work.
As we use VertiGIS WebOffice we have already Tomcat running. GeoServer installation is basically downloading the right file (Web-Archive), unzipping it and copying the resulting WAR file in to Tomcats WebApp folder and finally restarting Tomcat. Second step is to set up the WMS service - again straightforward as GeoServer comes with an easy to use and self-explaining web-interface.
|Map project in epsg:2056, WMS service provided in epsg:25832 but reprojected by GeoServer|