Well if you want a full 100% extra weight allowance against other machines then it shouldn't be easy!

It's wrong to think about it as a vertical and lateral motion problem as Craigs machine actually has both movements. You should think about it as axis of motion. Nothing wrong with using crank mechanisms however because there is only a single mechanism, it can't have 2 seperate axis.

For a true walker you need to have 2 demonstratable axis that are completely independent of one another. It then becomes a challenge syncronising these two axis to give you the walking motion.