Make ligatures "monospaced" and use complex character substitution

The ligature characters used in Monocraft replace multiple characters and so are bigger than the other characters in the font. This causes the font to not technically be classified as a monospaced font on Mac. Fonts like Hasklig and FiraCode use complex character substitution that replace the characters with ligature glyphs that go beyond the bounds and spacer characters to match the width of the original character set. I am not sure how to reproduce this in FontForge and it seems to be complicated, so any help on how to replace multiple characters (like "===") with other multiple characters (like "[SPACE][SPACE][LIG]") would be appreciated!