Well after the frustration with the affiliate program yesterday, have to give them props, we did manage to establish proper communication, and they've subsequently set up a new certificate to allow HTTPS to be used on the domain. Do have to do some testing to make sure that promo content still works as expected when delivered via HTTPS, but it looks good so far.
Of course this all happened
after I had spent some time writing the custom HTTPS proxy.
But I did manage to get that working, so my site including promo iframes is now 100% HTTPS and is no longer tainted with warnings about insecure content. My referred hit counts are already going back up.