Maybe Singh has tried all the possible trenches and settled on this one, but I would be tempted to try something else. It seems to me that more of the mixture could be jetted toward the plug if you were to machine the grooves in a shallow figure "3" pattern. Take your last picture and lay a 3 on the head deck, with the middle arm of the 3 extended into the chamber. Use a large enough 3 so that the top and bottom arms of the figure would extend to the top and bottom of the picture. It seems you would pick up more mixture from the outer parts of the deck instead of losing them in a thin sheet to go wherever. I can visualize a router set up in a track fixture and mounting a carbide ball end mill. Alternately, how about a "T" laid on it's side with the ends of the top bar of the T machined very close to the lip of the flat?
Some of my stuff might sound nutty, but I'm just trying to help.