In the SuperKOM system, we are doing something like what
you describe.
The three characters "***" indicate start of inverse video,
a second "***" end of inverse video.
In the same way, "###" indicates boldface.
The single character "_", if followed by a letter, starts
underscoring, which continues until the next single "_".
If you want to include "***" in your text, and have it
show up on the recipient screens as three asterisks,
you can do this with "**######*". In the same way,
you can get "###" on the screen by "##******#".