The grease plug has worked for me every time I used it.
And it is true that the valves must be in closed position in the cylinder that you wish to grease in order for it to work.
I usually cut the cam chain as I always replace them in unknown engines , then remove the cam. Some guys want to salvage the timing chain for some reason, in which case , backing off the tappets would be the best they can do. while doing that ,they may also check for valve retraction to be sure a valve is not frozen and hanging out in the cylinder where the piston can damage it. And proceed from there.
I have been toying with an idea of head removal first to avoid valve damage for sure and get a visual of real condition and some cleanup time on the piston tops. Drill out 1/4" flat stock to mimic the head gasket rods only and a grease fitting in center of each cylinder. Apply head gasket, drilled flat stock, extention tubes around the head rods, washers,and bolt it all down,then hit the highest pistons with grease equally...zero damage is what I am after here. I figure once the flat stock is drilled , this should be a breeze each time it's used. I'll try the next 550 that way and report the results.
Meanwhile, Chewy , I have your address and will mail you out 2 grease plugs tomorrow. RB