Including .notdef in Features?

Is there a way to include .notdef (undefined characters) in a class without the following error message?

No. Why would you want to put the .notdef into an OT class?

Hi. I am making a placeholder type for layout design purposes, where the main idea is to create a line that replaces any character, so I am trying to find a way to include any glyphs that I have not made instead of it displaying the missing glyph character.

Looks like this, so I am using positional/contextual alternates as well.

Any suggestions?

Cannot and should not be done. This is not what .notdef is intended for. Otherwise there would be no way to tell if a character is not supported.

For your project, simply try to support as much of the BMP as possible.

Ok, I see. Thanks