Ok,
I think I’ve figured it out :
/ip firewall mangle
add action=mark-connection chain=postrouting comment="Mark Conn for dropbox" content=dropbox.com disabled=no new-connection-mark=dropbox-conn passthrough=yes
add action=mark-packet chain=postrouting comment="Mark Pkts for dropbox-conn" connection-mark=dropbox-conn disabled=no new-packet-mark=lo-prio-traffic-pkts passthrough=no
and
/queue simple add name=wan_conn_limit interface=ether1 max-limit=256k/256k
/queue simple add name=lo-prio-traffic packet-marks=lo-prio-traffic-pkts interface=ether1 parent=wan_conn_limit priority=8
Should do the trick (found the info here : http://aacable.wordpress.com/2012/01/05/qos-with-mikrotik-reference-guide/ ). I just did a test with the first line of code in mangle and the connection gets tagged properly in the connection list.
Hope this helps someone else.