I've also been interested in how to use magic circles in the game (outside of thaumaturgy).  After looking through the book I finally found a margin note from Bob on page 230 that says that magic circles could be considered like thresholds (an illustration also applied to running water): "By this definition, magical circles could be considered thresholds, but I don’t think you really need to worry about that for the purposes of the game."

The comment is slightly out of place, and I think should be at the bottom of the page near this line: "But other things serve as thresholds as well. In the broadest sense, the term “threshold” may be given to any metaphysical barrier that impedes or blocks supernatural power from passing from point A to point B."

Looking at the effects of a threshold (block, suppress, source of harm), this seems to work mechanically although going with the same Fair +2 value as standard thresholds and running water does not seem to give them the same oomph as in the game.  Maybe because of their fragility they could have a value = to the user's Conviction score but no stress tracks (magic immunity, but automatically fails against any physical attack regardless of whether it would do stress damage to a target normally)?  This feels right since Conviction represents and individual's strength of belief, which seems essential to making a successful circle.

Just my two cents.