Re:keeping the lens clean - It kind of looks like the focal point of the beam might work in your favor. Using a stratified charge, the focal point (ie spark) may be in the fuel-rich zone near the piston head. In that case, the lens might be in an extremely lean zone. Besides, since the engine was already running leaner than standard, carbon deposits shouldn't form. If you look at pictures of conventional plugs running in a too-lean environment, the plug will be very clean.
Melted tip, sure, but clean. Other deposits tend to be symptoms of other problems (oil leak or contaminated gas).