Has anyone used the Tomz Knife Massager to sharpen plane irons?

I have not, personally I use water stones and the Veritas honing guide. However I only use that because the Work Sharp 3000 hadn't come out yet, which the Tomz seems quite similar to. If you could rig up some kind of guide to be sure you're getting a good straight bevel and you have the right grits it should work.

Thanks. I've constructed a platform and have a honing jig to hold the iron for the correct angle. I can't decide what I should do about the back. Any suggestions?

I assume you mean the back of the iron? It's flat? The way I hone a knife is to lay it flat and tilt it up about the thickness of a dime. I'd do a plane iron the same way, it's a wide knife blade.

Yes - the back of the iron. It looks flat, but I don't know how I would test it to confirm that it is "dead flat".

I know that people sharpening plane irons on stones use "the ruler trick", which sounds close to what you are saying: lay the back of the iron flat on the stone, cutting edge within 1/4" of one of the long edges of the stone. Then on the opposite edge of the stone they insert a very thin metal ruler under the iron. That has the effect of raising it a few degrees.