You're not alone in caring about this...
A few months ago the OpenWrt gained the ability to match and set DSCP marks in the firewall with commit https://github.com/openwrt/openwrt/commit/03e5dcbf10b1f67a463735efd8db3386f91473fd. I don't think there's support yet for this in Luci, but hopefully soon 
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You could also dig through the specific fw3 commit for more details.
For the current stable versions, you could create custom user firewall rules in the GUI, or follow @jow's earlier advice:
Cheers and take care...