I used to build wood/epoxy boats for a living, and I've used epoxies for building and repairs now since 1976. Epoxy is wonderful stuff and will seal wood really well. Makes a great base for varnishes and the clear coat for cars is really tough stuff. It's a two part and isn't cheap by the way.
You MUST be certain you can maintain the finish. IF you let it get bad, and let the sun get to the epoxy so IT has to come off, you are letting yourself in for a major job. And that's the real beauty of spar varnishes- they can be touched up. The clear coats ( and polyurethanes) don't lend themselves as well to touch ups.
SO yeah- great idea provided you stay on top of the topcoats and don't let them go. Means touching up nicks and dings as soon as you see them, which of course you should do anyway so sun and moisture doesn't get under them. If you can't see yourself doing that, then don't put the epoxy on there.