Ugly a sin but take a look at the Biria EZ Boarding series bikes as ones that look like they could be converted to belt drive with no frame modofication. They have high set chain stays which run above the upper run of the chain line.
http://biria.com/bicycles/eb/eb_superlight_8_black.jsp
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/