I believe these three threads are the best hints for others on this subject:
One
Two
Three
Using that knowledge and Mathias posts in here, I have a panel_class.dat file now, which spawns new tabs for each class bootstrapped to the hero. Also chatting with RavenX, the way to customize the panel is to have one panel definition and express tags to turn on/off different types of class features (spells, bonus feats, etc).
I'm making progress understanding this.
I've included a photo and a zip of my panel_class.tab.
The Class component is unique and has identity tags:
<identity group="ClsTabLive"/>
plus:
<eval index="1" phase="Level" priority="1000" name="Add Class Tags">
<![CDATA[
~ Assign tags to the Hero
perform forward[Class.?]
~ Activate my tab, now that I have levels in this class.
perform forward[ClsTabLive.?]
]]></eval>