Just in case any of the rest of you want to try this combination, I'll tell you how my experiment fared.
When I finally got my nerve up to try the two tools together, mechanically all was fine. But what happened was that the weight of the wall hanging sized quilt created enough of a drag on the circular attachment to make it hang up, creating really tiny stitches, despite the efforts of the walking foot to keep things moving along.
The other thing that happened was that the sliding gauge part of the circular attachment kept moving on me, which distorted those perfect circles I was hoping for.
Of course, some of this could very well be due to my inexperience with machine quilting. Perhaps tape or something could possibly stop that sliding gauge.
We shall see.