I’m working on in-progress typefaces that currently only has capitols. For the sake of convenience I’d like to assign the upper case glyphs to the lower case as well. Is there a way to enable a substitution rule that’s enabled by default rather than as a feature that must be user activated?
I think this is not possible. You can only substitute glyphs that are present in the font.
The only option is to duplicate the uppercase. If you use components, this will not be a big problem.