Here is one example that does not work because downloads break:
http://forum.mikrotik.com/t/how-to-use-transparent-web-proxy-with-2-internet-connections/2257/1
Here is another example of unanswered questions regarding traffic originating locally:
http://forum.mikrotik.com/t/mark-routing-locally-generated-traffic-like-web-proxy/4458/1