Ok so the high level goal is to be able to inject extra data into a message (which could include a signature). Add in a magic string and then a browser extension could detect it and decode it (maybe add a little badge inline). From the API perspective you'd want a standard way to transform an arbitrary block of text and data you want to attach to it into a text+data blob that still looks and feels like text to humans, and then for the reverse, regex for the magic string and decode from there to the end of the message to get the original text and data out separately.
One way would be to place N zws (zero-width spaces) between each original character, and treat each block as a digit which encodes a number. This could work for large original texts, but it would be clunky and very low bandwidth I think. E.g. if "." is zws, you could encode "fox" and the number 123 as "f.o..x...".
Better I think would be to create an alphabet with several of the zero-with characters and put the whole encoded number somewhere where it's unlikely to get trimmed or mess up line breaks (probably near the end in the interior but on a word boundary next to a space).
The hardest part would be making a transformation that wouldn't simplify it too much but would still be resilient enough to the transformations done by many forums like markdown/bbcode/trimming that the result could be perfectly converted into a PGP message. Maybe include some error correction?