If you wanted to use it just for braising or stewing (bottom heat only), there is a really simple solution. Just get some old tuna fish cans (1 or 2) and rest the DO on top of them and that will give you the clearance to get the coals under the DO.
For baking where you need primarily top down heat, but you have a domed lid, it's a bit tricky.
What you could try is get a cake pan like this (click this link). Now, use a can opener like the old military style openers or a dremel tool or other cutting wheel and cut the bottom out of the pan. This will give you a metal ring.
Place the ring on top of the dome, and then stack your charcoal inside the ring and on top of the dome lid. The cake pan ring will keep your coals from falling off.
For more stability, get some old wire clothes hangers and then punch two holes in the cake pan ring. Now make some "hooks" that you can attach to the cake pan ring and then fasten those to the dome lid's loop handle. This will keep the ring from sliding off.
The only problem here is that it's going to be really hard to take the lid off and check your dish. So, you'll need to be really sure of your recipe, cooking time, and coal count. Load the coals into your makeshift ring, and then leave it alone till the dish is done.
Alternatively, check eBay and search for "dutch oven lid" and look for a flat style (or one that is flatter than the one you have) that will fit your unit.
One other thing I've seen some people do with some success is to make a ring out of some heavy duty aluminum foil. Just tear off a long strip and then roll it up into a "rope". Now fashion that rope into a ring and put it on the domed lid of your DO. Now carefully stack the coals on the lid starting at the aluminum foil ring so that they rest on it, and then move inward after you've made a complete circle of coals.
As long as you don't move the DO, bump it, try to open the lid, etc, the aluminum foil ring will hold the coals in place.
1. Direct coal contact with the inside will destroy your seasoning on the inside of the lid. Something to think about.
2. You won't get as tight a seal, but that won't really matter for baking, but would make a difference for braising and stewing.
3. It's going to be VERY hard to get that hot lid back off the DO since it is upside down and there is no handle to grab.
Since the lid is domed, if you use it upside down, the coals will tend to gather in the middle of the lid and create a hot spot. To avoid this, get an old tuna fish can and put it in the middle of the lid to more evenly disperse the coals.
Also, how deep is your DO, and how deep will the handle of the lid protrude down into the DO if it is on upside down? It's possible the cobbler could rise and hit the handle of the lid if it protrudes too deeply into the DO.