Apparently, the Unicode Standard now requires processes that support RTL
characters to support mirroring of even characters that do not have a mirroring
pair. Such a list is available in the commented part at the end of the following
Forgot to complete the bug.
... This may be only be possible through the 'rlta' feature of OpenType:
So we have to add the 'rtla' feature to glyphs that currently have mirrored equivalents (first part of the list) and create and add it to those that don't (second part of the list)?