I'm going for a long shot here: Have you checked for a GAME.GOG file in the installation folder? This seems to be the Image file many games on GoG are using. However this might only be applied to DOSBOX games. I just discovered this so I'm not sure. I didn't know DOSBox could mount images - but I guess it is not that unusual.
If there is that suspicious file you can make a copy of it and add a ".iso" ending. This way you will be able to mount it with any mounting tool.
Personally tested with my TB1 release :) ;)
Nope, there isn't. No file with the right size either.