I've seen several systems that wrap the ssh binary to allow different agents to be forwarded to different hosts. Personally I think this is unwieldy -- it really should be something which is built into ssh directly.

Running multiple agents is also a bit ugly, especially if you are trying to consolidate your keys with an agent integrated with your desktop environment, which I think is the most common use case.

FWIW my proposal for fixing it is https://github.com/openssh/openssh-portable/pull/233 but it isn't the most elegant solution either I guess. It doesn't seem to have picked up much interest so I don't think it's likely to ever be merged (at least in its current form) which is fine. Hopefully some tamed version of agent forwarding appears directly in openssh someday, either as a simple key filter or something more complicated like guardian-agent