I have a need for setting the 802.1p tag based on the DSCP tag. I.e. basic DSCP-to-CoS translation, like vconfig set_egress_map does on plain Linux.

It seems like there are two possible workarounds for achieving this on RouterOS:

I can use the bridge chip to do the mapping. Unfortunately the platform is RB1000 which doesn't have a bridge chip, and the VLAN's are double tagged, so the bridge chip probably can't find the DSCP values anyway (they'll be offset by 16 bits because of the extra VLAN tag).

I can route to a bridge interface instead of to a physical interface. This probably works but it's a lot of configuration for 500 interfaces... Also, it seems like it could hurt performance.

Would it be difficult to expose the set_egress_map function in RouterOS?

Just want to confirm that this works. As for QinQ, new-priority sets the priority of both vlans.
The manual isn't very clear on the difference between from-dscp and from-ingress. Can anyone clarify? I imagine the difference is something like this:
from-ingress keeps the ingress priority regardless of dscp value (e.g. if the other device has a different cos-dscp mapping), where from-dscp uses whatever cos-dscp mapping routeros has. Am I right?

yes, 'from-ingress' uses ingress packet .1p priority, w/o any mappings

Thanks Chupaka! That's brilliant!

"For every complex problem, there is a solution that is simple, neat, and wrong."

Apparently for this particular problem there is also a solution that is simple, neat, and correct...