Hi Jim, it may work, but the first issue I can see is that a barrel is a round container, and are used to hold contents that push outward equally on all surfaces. A Box will have bowed-out sides if holding a liquid.
The same is true for a box under a vacuum, I think the sides will bow inward if the vacuum is excessive. This will depend on the thickness [stiffness] of the metal your hopper is made of. You will need to have coal in the bottom of the hopper, or else the vacuum will pull air and ash form the top of the grate into the hopper. You may have to close off the flue to effect a sealed chamber in the Hopper...
I'd give it a try, just a close fitting piece of 1/2" plywood with appropriate holes for the hoses. Try to find smooth wall hose for the rice coal to flow through, corn is round and light, and would roll/bounce off the corrugations inside a normal shop vac's hose, Coal is heavier and sharper, and may hang up on the bumpy surface.
Also, keep the vertical lift distance to a minumum, or the vacuum won't have a chance to lift the coal..
Let us know how it works, if done right, it may eliminate some coal dust in the house. I assume this is why you want to try this??