Another hireling Loyalty check optional rule occurred to me - loose and ominous table talk.
Examples of this are:
"It's okay - if they die, we'll just make them into zombies. Zombies are better, anyway, they're stronger and will follow any command."
"The good part is we get 40% value of their armor and weapons after they die!"
"I suppose we can just have the hirelings run in there and then shoot into melee and see if they're immune to non-magic weapons or not."
"The hirelings can run in holding alchemist's fire and then drop them. They can stop, drop, and roll out the flames before they take serious damage."
You know - openly discussing the hirelings like they're not there, and making suggestions of uncaring nastiness.
You might say, but doesn't Team Evil do this all of the time?
Yes, Team Evil probably does. But you hear about mob figures rubbing each other out and rebellions against the dark lord and how you can't trust your evil backstabbing "ally." The downside for being evil, ruthless, and untrustworthy is that people who associate with you are probably the same. Or they're fools, which is why evil henchmen aren't widely known for their faithful and effective service. The smart ones backstab you and the dumb ones fail you through stupidity, and the zombies just get chased away with True Faith.
Team Evil has ways of making you cooperate, or join them, but not generally of making you Loyal.
But I digress.
Ominous Table Talk
If the PCs make the hirelings feel like future zombies carrying re-salable goods instead of actual hirelings, use the following rule.
Make a Loyalty roll. If the hireling passes, there is no effect. They're unimpressed by the commentary. If the hireling fails, reduce Loyalty temporarily (for one day or one delve) by -1. If the suggestion seems to be followed up on, make this reduction permanent.
This is cumulative! You can rapidly drive a moderate to low Loyalty hireling to desertion or refusal to serve in this manner. Failure to obey or desertion happens when it's most convenient for the hireling. Remember to apply Obey Under Protest.