Hi everyone.
The unimesh workaround does work now! I don't know why it didn't yesterday.
@Defaultguy. I didn't know you and Nerd3D are from the development team. Thank you for your quick investigation and feedback.

@ Y-Phil. That script will be very handy. Do you think it's also possible to enable all the "light emitter" checkboxes of multiple model's body-parts in a scene with a script?
I've never given Python scripts much attention unfortunately.

@Parkdalegardener. I misunderstood "Looking at P12 in another window" in my previous comment.
Indeed the function shouldn't be of any relevance with SuperFly because it ignores it's setting. So the function is kind of outdated. But because it has issues with some models under rare conditions, it is relevant.

@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.

I've did some extra tests and it seems that the light bounce explanation is correct, thank you I didn't know that.
Though deselecting "light emitter" also disables the self illumination and that's where I incorrectly interpreted it's function.
That doesn't explain why SuperFly goes wrong though, because it ignores the function and includes geometry with "light emitter" disabled in indirect light anyway.

@Ghostship. I've checked that. There's no difference in bounced light between the two options.
The ball in the centre is the emitter. The ball on the left has "light emitter" enabled, the ball on the right has it disabled.

The "light emitter" option already exists in many older versions when there was only the FireFly render-engine. Indirect light in Firefly
gives quite poor results and takes ages to render when you increase the quality to acceptable results. I assume they added this option
so users can include and exclude specific ambient materials to emit or not emit light light and prevent unnecessary processing time.
At least that's the the way I've used it on some models. There should have been a reason for the developers to include the option I'd say.

Oh wow, thanks Y-Phil. You're right, it's actually Poser that goes wonky on it ;p
The issue seems to have started with a recent update. I do hope the developers find a way to fix it.

In addition. One quite easy fix for this (if you don't own an older Poser version or runtime editor tool) is to open the CR2 file with a text editor and rename the obj filename manually. Don't forget to make a backup. The obj reference file is listed twice under for example: figureResFile  :Runtime:Geometries:2nd World:Airplane C-172:C172_Airplane.obj
So use the search function of your text editor and rename both file references.

This issue is one of the reasons why I still use Poser Pro 2014 to save my models and products.
In poser 11, if you for example rename an obj file and load the model, Poser can't find it and will ask you to locate the obj. If you do that the model will load perfectly by using the renamed obj file. However if you save/overwrite the model it still saves the old obj file name and/or path.
The other reason I still use Poser Pro 2014 to save my models is that when you save a rigged (character) model in Poser 11, the programmed master parameter control dials won't work for DAZ users. Loading and saving/overwriting it with Poser Pro 2014 will fix that issue.

Thanks Phil. It's probably my favourite! Pity it goes wonky though ;)

Incredibly tragic, so young... My condolences to everyone.

Attached Link: 2nd World store