Wow. A #10 screw is nominally .190" diameter. That doesn't leave much meat on the lugs near the edges--probably about .065" based on a quick measurement here. A standard OD for a flat washer is .320" diameter, and that will go just about tangent to the outside of the lugs. But the problem is in "coning" the washer. They are pretty thin and you can see from my scaled sketch that without the tubing, you will be bearing on the edges of your metal-filled epoxy. That would work for a while, but it wouldn't surprise me if the screws kept losing preload as the epoxy creeps underneath it. Doubling up the washers would help; a thicker single washer would be better. I had a pair of steel pieces that were like a long rectangular "washer" that had both holes in it. That worked well. It's not the same as an engine clamp, which, as you mentioned would be another choice. Sleeving it with tubing such as the K&S 5/32" x .014" wall in brass would sure help if you can dress the tubing flush with both sides of the lug.
In the sketch, I penciled in the tubing and filler bond on the one side to see what that would look like.
If you look at the load path for the clamping force, it develops under the screwhead, is transferred thru the thickness of the washer and then--if the tubing is dressed flush with the top of the lug down into the engine mount. For a wood mount, the contact stress (compression) of just the end of the tube would be very high if there was no bond effectiveness or load transfer from the tubing thru the bond into the lug. So the bond joint is in shear, with an approximate area of .096 in^2. If tightened to about 5 in-lb of torque, resulting in approximately 240 lb of preload, the shear stress on the bond is around 2.5ksi. The JB Weld should be good for that as long as it doesn't get too hot. So should you use a sleeve? Yes! Should you clean everything really well and bond it in? Yes!
One more comment regarding the JB Weld material. I have found that it is more long-lived in a repair around fuel and oil if the material is left with the "skin" on it. If you cut or file the stuff after it cures, it seems to open up pathways thru the filler material letting the fuel attack it more aggressively. I have done plenty of venturi repairs, sleeves, etc and they hold up better with the skin left on.
Dave