WOW - I just had overlooked that.
Should have known additive operations are not allowed at non 1.0 gamma....
* Christoph C. Feldhaim slaps forehead
Additive operations are 100% allowed at non 1.0 gamma, but they will not exactly mimic what you get in a single exposure (I did a comparision one day but cannot find it now, will repeat it again tonight just for curious). I was just being strict here though since the results between adding images using 1.0 gamma profiles and any other arbitrary gamma will not differ much; in fact you may even prefer those obtained using gamma profiles. In addition to that the 1.0 gamma images must have been neutrally developed (not contrast, bright or saturation adjustments yet), then added, then processed.
This is achieved how GL?
Just need to convert images to linear versions of your profile (sRGB, Adobe RGB,...). You can build a linear version of any profile straight in PS. I explained how in the DCRAW Tutorial
, in the 'COLOUR PROFILE' section near the end. But as I said, this is just to be strict, I don't think the mess is worth.