Good premise. May be tough to execute, as you point out. On the other hand, introducing oppression or superstition (especially oppression through superstition) is easier than it seems at first. It could start simply: "women shouldn't use magic because it's in their best interest" during disaster/war/calamity X. Initial reasons are forgotten if calamity X lasts a generation or more. Is it a cop-out to suggest that the magic is sufficiently advanced to allow for mind control or memory modification? Are you going for the historical crusades or even their time period or what? This planet even?

A traditional tune you might enjoy: http://www3.clearlight.com/~acsa/introjs.htm?/~acsa/songfile/JACKAROE.HTM