I can't give you a diagram, but instead will give you an idea of how it must work.
First, we know that hot air rises, carrying smoke particales and unburned gasses with it. We also know that a grill cooks by radient heat, or in other words, infra-red radiation.
We know that in a good brick oven, the fire brick absorbs much of the heat and radiates it evenly to the food. It also captures heat to exchange to the food through convection.
Brick ovens, grills, and barbecues require heat, which means they require fire, which requires air, and some way to control the amount of air-flow.
To satisfy the requirements of a multi-purpose unit, I would build the basic structure of fire-brick, in a semi-cicular form, like a pizza hearth. The hearth would haf an open semi-circular, or rectangular attachment directly over and around the fire-pit. I would also have niches installed for any desired attachments such as a rotisserie, and tight cover for the pit. Of course, it would be very handy to have a removable ash-catcher at the bottom to facilitate ash removal. There also needs to be a cooking grate, and a place for an insertable water container underneath the grate. The whole unit should be high enough to provide room for a fire grate between the ash catcher and water container. If possible, the fire grate should be adjustable in the vertical plain so that it can be moved closer to the food when the water pan isn't required.
The main hearth should be an enclosed structure with a tight door. You should also install an accurate temperature gauge that measures the inside temperature with an attached probe. The chimney would be built at the top of the smoke-hearth oven. Multiple racks or hooks with removable trays should be installed into the smoke oven.
The oven and grill would be attached either by a common hole between the two, or by an air-duct.
Air controll would be provided by closing the grill cover (this cover should be vented, and more vents should be built into the grill sides), and using baffles in the chimney to partially stave the fire of oxygen.
For shape, think of an inverted igloo (the grill) attached to a large, enclosed brick oven, standing on a pedestal, and with a chimney on top. The different parts would be built side by side.
With a domed lid, the grill portions would work much like a Webber Kettle babecue. And the oven could be used at many different temperatures by controlling the air flow, giving it the versatility to cold-smoke, hot-smoke, roast, and bake. And if you really wanted to get fancy, add a motorized mister ot the oven, operated either with a timer, or toggle switch to add moisture to the oven for the smoking process.
You could roast hogs in this thing, or make jerky, or even bake a cake. You would just need to adjust the fire size and air flow. And the thermometer would remove the guesswork.
I would suggest giving this post to your mason-working freind and draw up a design you both can live with.
Just keep the physical properties required to perform the various tasks in mind. The design will come to you.
Seeeeeya; Goodweed of the North
