You can actually use any double character for this. Go to:
Settings->Preferences->Display->Text Barring Character
to change it.
I had to do this once when re-engineering a design for a customer who required that the net names be kept the same and some started with the double underscore character.
Be aware that this is a global change and not in the design technology of the specific project, so it will affect all designs until it is changed again. Quite a few years ago I did put in a suggestion to have this feature moved into the Design Technology file so that it became project specific, but nothing came of it.
------------------------------------------------------- Birthdays are good for you: the more you have, the longer you live.