... correction, I can also train Shieldman, but not Henchman nor Torch Bearers.

Also, there are odd effects when I use Spellbooks on these units ... if I give them a spellbook they start with the simplistic Adept skill at, they'll tend to get the full first level spells but won't be able to get more advanced ones.  If I give them spellbooks for something they have no Adept skill at, they can progress normally.  I will try to show a screenshot of this.

Just tested this in-game, first with a custom Altar faction from my current playthrough, then with the stock Altar faction. Henchmen, and all of their pre-made templates, are working in .92.  You could try to use ctrl-r to max your research as a test, then see if they are buildable.

As for the spellbooks, the spell schools for henchmen are very limited, but you can give them the full complement of spells via the Decalon or found spellbooks.

How do you design your own henchmen?  They aren't normal units.  They can't be built in groups ... they are essentially trainable Champions.  I'm building them so I can level them into Mages (or other professions), equip them with full spellbooks, etc.

Definitely some merit to this idea, imo. And yea, don't use the stock henchmen, making your own specialists is so much better, and a lot of fun. Try making a commander, a mage, and defender for your main armies, last game I had several over 20th level.