That is a clever thought, but I think that would be a "tough nut to crack". The "overlays" you are talking about are only applicable in the "browse map" mode. Unfortunately, even in "browse map mode", the unit still locks to the underlying system map road system. The only way I have found to get around this "road lock" is to run some third party software (
http://www.webazar.org/tomtom/offroad.php?lang=uk) that over rides the systems default navigation settings. It is a major pain in the neck, but it can be done. This third party software does not work on all models, and it will not run under the latest firmware update (at least so I have read). An easier way around this is just to invest in a hand held nav unit (Garmin has some great ones) that are not "road locked". You can be anywhere on the planet (or even above it), and still know where you are (or have been) to within about 30 feet or so.
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TT ONE XL APP 7.162
tomtom.mnu, Reverseitn, Tripmaster, Offroad, MenuTT
USA_and_Canada V675.1409
Using a 2 GIG SD Card for "emergency" backup