Whacked the hex key with a hammer. The key moved. Nothing else happened. Tried a few more times mostly due to frustration. Same result. Chuck is still on there.
Tried making a poor man's cone wrench out of 0.125" mild steel, with a hacksaw and a file. Filed the wrench until it fit. The nut dimension was different than expected. It ended up with 16.9 mm. Initially I estimated it at 14 mm. Of course the mild steel wrench wasn't up to the task. The wrench distorted and got stuck. The clearance between the chuck and drill is 0.140". Took a while to extract the wrench from in between the chuck and the drill body. Chuck is still on there.
Learned 2 things, 1) making a wrench this way is not much fun and 2) I can buy a small set of HF metric wrenches for less than the cost of a single cone or tappet wrench. The tappet wrench that I found in 16/17mm are 0.181" thick, so I would have to grind to fit.
Gee, this darn chuck is a royal pain. I never expected this much "learning".