Rhionon, you could try teaching the JCMs with the dependency editor. Put the figure in zero pose with the JCM off. Bring up the dependency editor for the rotation that drives the JCM in the clothing. Start Teaching. Pose to where the JCM will be at full strength. Activate the JCM. Pose back to zero position. Deactivate the JCM. Stop teaching.

Make sure "Include morphs when conforming" is checked in your clothing figure.

This method drives the JCMs with keys. You don't have to code this stuff yourself, unless you want to get fancy with the valueOpTimes function which can't be accessed through Poser's UI. You can even stack JCMs and teach them to start where the last one leaves off. 

The one downside is that people with versions older than Poser 8 won't be able to use your stuff.

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