I have been struggling with Curtain wall phasing for a while now. This solution doesn't fix the problem of wanting to phase the mullions, but it does help with the phasing of the infill panels (whatever they may be).

The trick is really just not having a panel at all that is hosted in the curtain wall. Set it to "Empty Panel."

Next, create a generic model family for your Glazing. Make sure it is non hosted.

Then, you can group your frame and glazing together, and you can still tab into the group and select the glazing and tell it to be demoed.

Finally, you can use another generic model family for a door or another custom glazing or something to infill it with.

Hope this helps.