Hmm, Signature Flexibility looks like it might be very relevant for me. Is this just to allow redundant ECDSA signatures, or could this allow adding signatures of other types?

I'm looking into piggybacking hash based signatures into transactions for a proof of concept ("Adding extra custom_binary operation to transactions" section). If I could add my own hash-based sigs as redundant signature that would simplify my attempts to piggyback hash based signatures quite a bit.