That's a neat idea, to use oleganza's scheme as a threshold signature. However, you can't simply execute this scheme unblindedly. If the blindsigner knows all of a, b, c and d then he can determine the entire private key and create signatures all on his own. But if he doesn't know all of those, there is no way that he can be sure of what he is signing (or even of the public key that he is signing with). So this is a threshold signature in some sense... but a very unnatural sense.