The Nashbar steel fork is a good one in 700 c and the LHT fork is available in both wheel sizes. I'm currently making my own tandem weight fork, since an unloaded tandem can have less front end load than a touring bike. The stays are massive and heavy, and it has me a little worried, but it should be good on the number.
There is an old saying that the fork is the bike, which i guess mainly means that the fork is like a tuning fork and is more sensitive that the rear triangle because the latter is braced. The variation in stiffnes options in the fork is pretty wide, and the variation in potential stiffness needed on Touring bikes with front racks is also pretty wide.
Other things to consider are what kind of bearing you need in the Headset for your load needs, and what kind of brake systems you want. In a custom situation this should come right down to tuning the fork for the actual set of brakes so that you can get the proper adjustment to your rim width and the right mechanical advantage.
If you want to carry a lot of load up front then you can go well beyond the standard rack mounting points. The exact formular will depend on whether you want to carry a load over the wheel or on low riders only. A lot of the better set-ups carry a high load as an option for something like a sleeping bag. Controling stuff that high up with stock mounts is not the best solution. And if you want to go further than stock you might as well clean up fender and brake options. What about running two front brakes like on a car?
So if you can get right into it you can probably find a case for a custom fork, but most seem to get by with some pretty basic gear.