Problems effectively using SPF with SRS

Let's say that I want to whitelist hotmail.com, because they publish SPF records and ratelimit outgoing mail on a per user basis.  My email address is forwarded to another email address, which is where I am implementing my whitelist.  Assume the forwarder implements SRS.

How can I make my whitelist work?  The From: header is not checked with SPF, so that's worthless.  Since the message was SRS-forwarded, I can't reliably tell who the original sender is.  At least, I don't think so.  Can anyone tell me how to do this?