@Parkdalegardener. The function and checkbox is still there in Poser 12. It's located on the model's property tab, not on the render settings panel.
The odd thing is that the customer who contacted me about the issue only ran into it recently, although using the model, Poser 12 and SuperFly in exactly the same way for longer. It seems to have started with an update. I can't check that because I've only purchased Poser 12 two days ago to see the issue. Because SuperFly in Poser 11 which I have been using all along doesn't have this problem. It completely ignores the function, disabled or enabled.
And indeed, mesh-lights and indirect light are brilliant in SuperFly. It's on a totally different level compared to FireFly's mesh-lights. But the issue isn't if it does it better, but why it goes wrong on basically a FireFly function while it ignores the setting.

@DefaultGuy. I've tried the Nerd3D skinning method workaround, but I'm afraid it doesn't work :(

Anyway, the developers team is already on it, so I assume they can fix it with the next update.