OK, here is some progress with this. I envisioned the penitent engines to be recyclable. Strap in a penitent and throw him of her into battle until redeemed through glorious death for the Emperor. Repair the engine and strap another penitent in. These machines would be functional easy to repair and crude and battle filthy. The scheme I chose uses only metallics and washes except for flesh and banners. I have used magnets to hold the 2 parts of the body together making it easier to assemble and paint. The arms are to be attached with wire or possibly magnets if I can find small ones strong enough. Here are the pictures.