The ability to train henchmen is not actually linked to the sovereign trait. It was just a way to visually display that all sovereigns of a particular race got that bonus.

The henchmen are actually linked to the race of Men directly, as you see in their unittype. 

I haven't tried having it unlock from a sovereign trait, but this is how I would go about it:

1) Create a new trait and have it unlock a tech

2) Create new henchman unit designs and have them unlock from that particular tech

As far as I can tell, Kalin is correct.  I've been playing around with this all day and the hang up to making it easy is that there's no "All" or "Any" variable for "RaceType".  So, in order to unlock Henchmen I had to do it for each specific race:

Modify the Techs_Amarian.xml file's "Henchmen" entry to eliminate any race requirements.  This should make the Henchmen tech show up in the research tree for any race.

Then you have to copy the Generic Henchmen and or the specific ones (Bard, etc) to a new .xml file.  Modify the entries to reflect the new race.  You will also need to add the new henchmen entries to the generic and starting units lists for the race.  Grab the henchwoman mod for examples on how to do this.

This method is pretty annoying, but I haven't found any easier way to do it yet.  Personally, I think something like the "%d" variable used for champion cost would be helpful.  Something like "%r" where the variable is replaced by the sovereign's race.

my .$02,

gs