There probably isn't anything like what you're seeking-- the problem is that these are very different conditions/requirements:
a) special diet out of preference/philosophy/belief-- Kosher, vegan, etc.
b) special diet required by intolerance (celiac, lactose intolerance, etc.)
c) food allergies
So that is three different populations. This forum/community is primarily a resource for the last of these, and for our community, the color TEAL and teal awareness ribbon are common, though far from widely recognized, symbols such as you seek.
I'd think that anyone IN one of those groups, however, should be expected to read the fine print-- in terms of signage and menu notes.
It is really great that you are making such an effort to make sure that your note doesn't go unnoticed.