I haven't found 4mm Stainless rod, but I did score some 5/32" which is within .002" of 4mm! I have 6 feet of it coming (about $15, so not too bad). If anyone needs some, I will sell it for $3 a foot including shipping. That should be about a break-even for me.

Has anyone ever tried letting the bulb pivot to drive a trim tab at the bottom of the keel to help keep the boat upright? I suppose spring centering and some dampening would be needed to keep the system stable.