Actually, with cloaking turned on, P202 washes 99.8% of referrers. Yeah, I made that number up, but it is very effective. If you're paranoid, have your outbound link perform a javascript redirect (window.location='offerurl.php') to the prosper link, which then performs a double meta refresh. Hope that helps.
Do you still mask your referrers on offers that make you reveal them *cough* Flycell *cough*? I thought about it but I'm a little scared to try it incase they decide to cancel all the leads from sources they can't track.
Just do a double meta refresh where the last redirect checks to see if the referrer is leaked. If the referrer is leaked have the last meta refresh send it to a clean page which, when coming from the referring url that got leaked shows a button saying "click here to continue to your offer", or "before continuing click this button to prove you aren't a bot", and if there is a different (or no refer) it shows a clean page.
That way if the referrer doesn't get leaked they get redirected to the offer and the advertiser doesn't know anything. Then if it does leak it is sent to a page which filters the user right to the offer and looks clean if the advertiser/network looks at it.