This one's just screams "too localized" and "not a real question". I'm trying to think of how this can be salvaged, but I imagine even if the OP clarifies his "obscure and silly purposes" it'll still be no good.
For obscure and silly purposes, I intend to generate an OpenPGP RSA key pair where a part (~half) of the private key is as I specify. Are there tools for completing such a task without delving deeper into the key formats?