After much searching, I finally found something on fleabay. It appears to be made in Taiwan by a company (I don't read Chinese) who makes a lot of replacement rubber parts for w140 and other MBs.
W140 Heater control Valve Repair Kit OEM Mercedes Benz W140 | eBay
Here's a pic from the auction, for when the auction link eventually dies.
Anyone used one of these before? Any U.S. suppliers of this, such as here on Pelican? Can't even find the valve in the parts lookup, search engine could use some help...