I've played enough with 2x2 round plates and 1-stud-wide plates to know that it's possible to attach a plate underneath a 2x2 plate and perpendicular to it, like so:
The same connection is possible using 2x2 round bricks instead, or using 1-stud-wide tiles.
An observation: each of the 4 wall sections of a 2x2 round brick/plate can flex slightly. This does not happen with the continuous walls in a square brick/plate.
Is this connection "legal"? (i.e.: Does this connection stress the parts involved?)
Has this type of connection been used in any official sets?
A word of warning: Do not attempt this technique on part 6143 "2x2 round brick reinforced": it won't work, and you risk denting your plates (as I just did). Do look for a flat wall section at the bottom of the round plate/brick.