The .tdb file is what instructs mapsource not to allow the routing for MetroGuide to be downloaded to the gps. If the .tdb file is edited to remove the restriction, then you can use mapsource with no other tricks to select and download the MetroGuide maps to the gps and get the routing and full indexing just like you would do with City Select or City Navigator maps. Attached is a patch file for MetroGuide Europe v8, once you patch the mgeu8.tdb file with it, you will be able to just select the maps in mapsource and download them to your gps to get routing and full indexing.
Download the patch program from here:
http://binpatch.sourceforge.net/
Then use the patch program and attached patch file to patch mgeu8.tdb.