The Basic Fantasy Role Playing Game divides hirelings into three categories: retainers, which equate roughly to AD&D's henchmen; specialists, which include savants (or sages) and skilled laborers and finally mercenaries. Regarding retainers, the BFRPG has this to say:
All retainers have a Loyalty score, which is generally 7 plus the employer's Charisma bonus (or penalty). The Loyalty score is used just as the Morale score of monsters or mercenaries is used. If a Loyalty check roll made in combat is a natural 2, the Loyalty of the retainer increases by +1 point. Note that a Loyalty of 12 is fanatical… the retainer will do virtually anything the player character asks, and never flee in combat...
At the close of my last campaign I was using, essentially, Tao of D&D's rules for henchmen found here. For Beneath the Broken Moon I'm looking to stick more closely to the standard rules, but that bit about fanatical loyalty is ripe for a house rule putting us closer to what Alexis is doing and what we had previously done.
My retainer rules will be as per the BFRPG book, with exceptions as noted here. Retainers begin with a base loyalty of 7. A character's Charisma bonus does not modify the base score but will modify any actual loyalty/ morale rolls. As above, any natural 2 rolled for a morale/ loyalty check will result in an increase of the base loyalty by one. Restoring a henchman from negative hit points will also increase this value by one, but a loyalty check will be required to see if the henchman desires to keep working for the player character. Having a brush with death, after all, does often cause one to reevaluate their life and reconsider their current circumstances.
If a retainer's base loyalty/ morale score ever reaches 12, the player controlling the retainer may opt to take it over as a second player-character. Each PC may only have one such henchman, but nothing is preventing a player-character's henchman from having thier own etc... provided the requirements above are met.
In this manner, having a henchman comes about more as a reward for play then as a reward for level achievement, but still requires that some objective criteria be met. Meeting them should be no small feat given that the opportunities one has to increase one's retainer loyalty also tend to put those retainers at great risk or evoke in the follower a desire to seek employment elsewhere.