I just saw this article about this building block set that's purpose designed for mechs. A quick google didn't reveal any mention of it in relation to Mechaton. Now I haven't read the rules so I don't know if you can build legal mechs with this stuff, but man does it look cool.

Asoblocks, at Thinkgeek.

They seem a bit large for Mechaton, but if you have a large table, I imagine it's a possibility. Certainly they seem much easier to construct for a lego-handicapped guy like me.

Now, being that those things are much less realistic than legos, you might want to paint some pieces to color-code their functions for the game. Use blue components for things that give blue dice and so on. I imagine that those are liable to get confusing if you only use one color and have to figure out which part is a gun and which is an antenna.

Lego has been cranking out a lot of larger scale mech-ish stuff. A quick glance through their catalog reveals that almost all of their recent lines are at least vaguely mech-able, the castle stuff notwithstanding, and a lot of it just IS mecha, period.

Of course it is all way too big for Mechaton, unless one wanted to up the scale, which, frankly, one does not.

The Asoblocks are pretty neat, though, especially for more Eva-looking mechs.

Just seven different pieces seems a little limiting. Too few for me. But on the other hand they got this weird organic look I like.

The manufacturer is constantly coming out with new blocks in Japan. There are waaaay more than just seven different pieces. A friend of mine picked up a couple of boxes for me in Japan, and I think i have about 16 different types of blocks. I'm still sort of getting a hang of how to build joints and limbs.