no can do with the Harken. they all require a H section track to slide along, which requires a flat surface.
I think this is incorrect. Check the Harken website, they state that curved traveller tracks are no problem...http://www.harken.com/charts/curvedtrack.php
Also, the original H17 Sports came with a Harken traveler track riveted to the rear beam, and that was a curved beam. And many boats use curved Harken traveller tracks for their self-tacking jibs.
In any case, the problem with the harken travellers is that the friction is so low that when you sheet in the main, you end up pulling the whole block stack to windward.
I've never really had much issue with the stock H18 track. It's a little draggy, by is adequate. You need to keep the track clean and lubed with silicone and it will operate fine. We don't work the main traveller sailing upwind, at least not on a constant basis. I may have the crew drop it down or pull it up a few inches, but it's more of a gross setting. Working the mainsheet and the downhaul as well as steering properly is the way to sail the 18 upwind.