Hey, I swear there was a new post here just now and it dissapeared before I finished the reply...
Anyway, what I meant was:
/ip firewall nat
add action=dst-nat chain=dstnat dst-port=53 protocol=udp dst-address=208.67.222.222 to-ports=5353
add action=dst-nat chain=dstnat dst-port=53 protocol=udp dst-address=208.67.220.220 to-ports=5353
add action=dst-nat chain=dstnat dst-port=53 protocol=tcp dst-address=208.67.222.222 to-ports=5353
add action=dst-nat chain=dstnat dst-port=53 protocol=tcp dst-address=208.67.220.220 to-ports=5353
So only requests to specified servers will get destination port changed from 53 to 5353.
What I don't like is that router will be left out (not just with this, but anything you can currently do), it won't be able to use these resolvers and you can't use it as cache. It could be done, but unlike Linux, RouterOS doesn't support dstnat in output chain.