TL;DR: Added IK rigging + walk and run animations in Blender, then used a 1D blendspace in UE4 to blend them according to character speed.
Blender’s Auto IK worked fine for quick tests, but didn’t give quite the desired results. Turned out I only needed to have one IK handle per leg- then set the chain length to 3- to get the whole leg to move correctly. By not parenting the IK handles to the root, they could remain stationary while the rest of the body moved.
One cool side-effect of having the extra middle part of the leg is that, as the running speed increases, the leg naturally expands and makes the mech taller, requiring less leg rotations, like how a car’s engine can switch gears. I can also imagine some type of spring or hydraulic system that uses the leg’s own forward momentum to bring it backwards. Dope stuff.