Converting a polyline works great.
Here are the steps, in case anyone is interested. (Or if I forget how I did it and search the forum again.)
Create a square polyline that is at least a little bigger than 1/4 of the octagonal hole. If a side of the octagon is S inches, that would be S * (1 + sqrt(2)) / 2, or S * 1.207107. In this picture, the sides are 36", so that is 43.4558, or about 43 7/16. A 50" square would be fine. Make sure the lower right corner is in the desired center of the octagon. (In retrospect, 36" sides may be too long for this room. You can plug whatever side length you want in the formula above.)
Use the break tool to put breaks on the right and bottom sides of the square polyline. Each break should be S * 1.207107 in from the lower right corner.
Pull the lower right corner of the polyline up and to the left until the pulled sides are parallel to the top and left edges, to make an L shape.
Put two more breaks on the inside corner you just made, S/2 inches in.
Pull the inner corner down and to the left until you create a diagonal of the octagon. It should be S inches long, and the horizontal and vertical sides should be S/2. Now you have a quarter of the octagon. You will copy and reflect this in a minute.
(Note: you don't have to make an octagon. For example, you could make a rectangle that follows the shape of your room.; put breaks in it so you can make an L shape; make a fillet curve in the inner corner; the pull the curve out into a convex shape.)
Decide how you want to handle the bump-up. If your room has a regular shape, you might raise the entire ceiling, or leave it as it is and plan to drop down the shape you're making. Either way, you would have to extend your shape to the corners of your room. Alternatively, you could create an inner room with room divider walls set just outside the hole you are making, and just bump up the ceiling there.
Now use the Convert To... tool to convert it to a countertop. Make the thickness 6" (or whatever you like). Set the bottom position to your main ceiling height, or the top position to the bumped-up ceiling height. If you like, add a crown molding to your edges. You could also set the material to drywall and pick the color you like, or do that later in the 3d view.
Now select the right edge, where it will join a reflected copy, and delete the crown molding there (if you added molding). Do the same for the bottom edge.
Make a duplicate copy of your countertop, reflected about the right edge. Shift-select both copies and make a duplicate copy of the pair, reflected about the bottom axis.
Now the 4 copies make a symmetrical octagon (or whatever shape you decided to make). Paint it if you didn't before.
Note: I converted to a countertop because I knew you could add molding. Not sure if you can with slabs, but if you don't need molding it should work the same. This is just a decorative effect. I imagine CA could do actual framing plans and materials lists.