Wazabear: what I was getting at is that you pick delimiters that can be used as reserved keywords. If your keyword is BEGIN_GLSL_FOR_OPENMW, it shouldn't inconvenience anyone. I mean, if someone does complain that their GLSL already contains that keyword, then they're just playing with you. You can point to them with a shaking finger and say "That was no accident!".
The only solution I see is to add special characters which absolutely mark end of a block, even if they are in a comment. Something like this, of course it would need to be made clear in the docs of this restriction.
I think that is the best approach. I approve this idea in my official capacity as a stranger on the internet.