What clear coat material would last as an exterior traffic surface?
Many epoxies are designed to have other ingredients mixed in. That's basically West System's hat.  The garage floor epoxy paints all allow grit and chips to be mixed in too.   

The problem is, epoxy is quickly destroyed by UV light, notably from the sun.  

There are several other 2-part coatings, and surfboard people are the experts: they work with every kind.  TAP Plastics is a retailer in the west (yes, a retail store you can walk in and buy this stuff) and they have extensive literature about the ups and downs of each 2-part coating system.