I think I've been successful!
Last night I placed some 5/16" diameter bars on my grates. They each lay between the 1/2" rebar I used for the grate surface, and rest on the grate axles This kept the stoker coal from falling through the grate openings, and worked well. If the bars do not hold up, I may try to find some #2 rebar, but it's a really odd size and few stores carry it.
(see this thread at Grate area vs. soot buildup
for a photo of my grates without the 5/16" bars I added). The 5/16" bars just rest on the grate axles. Still shakes fine.
I also took 2 of the 3 grates out of service by reducing the firebox. Layed a piece of sheet metal cut to fit on the rear 2 grates, and stacked firebrick on them. I then removed the "connecting rod" that ties the grates together for shaking. Thus when I shake the front grate, it does not move the rear grates. Total grate area is now just 20" x 4.5"
I installed 3 firebrick above the secondary air pipes, just layed them directly on top of the pipes. Also added a few bricks to the sides of the stove, resting them on the side brick retainers. I think the additional firebrick I used to reduce the firebox size really helped reflect more heat to the right area.
Also used some firestop hi-temp caulking to seal some joints in the secondary air piping.
Got a wood fire going (just a small amount of wood), and added some bit coal. Got about an inch of glowing coals covering the grate. I then filled the firebox with about 6" of coal, leaving one side open with coals still glowing. Opened the ash door and let it roar for a few minutes. When the stove skin temps above load door raised to 200F or 250F, I closed the ash door and opened the secondary air. Got the secondary burn from the piping above the bed (at least on the glowing coal side), and let that cook about 10-15 minutes. I then filled the remaining bed with coal, and shut the load door, and the secondary burn remained. It continued to burn for several hours. I peeked in several times, played with the air settings, etc. It did not seem to be fragile balance. Stove temps hovered between 350F and 400F. Door glass was pretty clean next morning, though stove temps were down some as, the coal bed just is not deep enough.
This morning I went thru a similar process, but of course instead of starting with wood, I opened the ash door, let things heat up, shook the ash down, and then added coal. Same story - got my secondary burn back, let it cook awhile, then filled the remainder of the firebox, and the secondary burn was still going 2 hours later. When I came home for lunch, I had a nice glowing coal bed, though the secondary burn was gone since all the volatiles were all cooked out.
I checked the chimney outside about 1/2 hour after loading, and I could not see any smoke, only some 'heat'. If I squinted my eyes, I might be able to imagine some smoke was there.
Few items still need taken care of are a deeper coal bed (some bars or firebrick stacked up in front of the lower half of the load door), a top load door, and a more sophisticated control for opening and closing the secondary air inlet. But I am so excited that I'm finally getting this to work.
I took some photos and video, but I was not too impressed with them. I will try to get some better ones tonight. The photos do not do it justice. I did not use a flash, and it was OK, but it doesn't look much like it looks with my eyeballs.
I imagine the results would be different with stove coal (1" plus). I may try it, but the stoker coal is nice as you get fewer rocks, which clog up the grates eventually. The smaller stoker coal doesn't seem to contain as many rocks and junk. I'll bet with stove coal, I'd maintain the secondary burn longer, and have more even temps.