You can do this. Just create a custom command with the URL pointing to some mapping service. For instance to use Google maps to search for a place:
http://maps.google.com/?q=Eiffel+Tower
More generically:
http://maps.google.com/?q={text}
This will replace the {text} placeholder with whatever text you have selected. Actually, there's a preset built into Marvin's command editor for Google Maps.
To use iOS' own maps app use
http://maps.apple.com/?q={text}. Don't be fooled by the "http://" prefix. iOS will still launch the built in maps app.
More info here:
http://blog.marvinapp.com/post/74510508547