@silverpill I can't really understand your example. The client doesn't have access to other actor's private keys, so it shouldn't be able to sign requests. Or you're thinking for the case of a client that is used by multiple users, *and* it stores private keys...
My clients generally use only OAuth2 for authorization to the service their users belong to and they don't do "signed requests" to other servers (because they don't really have access to the private key in the first place).