Is it possible to map the cos from a tagged frame into the metro tag cos field when it enters a dot1q-tunnel port?
The only option I see to set cos on a dot1q-tunnel port is to statically configure a value using the <mls qos cos <value> > commmand, this is with the 3750.
It all depends on the hardware.
For example 3750 Metro support copying inner CoS value to the outer tag. It is also supported by 4500s with SUPV-10GE.
Also this functionality is possible with ES20 cards in 7600.
Overall there is not much hardware that support it. The functionality you are looking for is called "ntelligent IEEE 802.1Q tunneling QoS"