My issue with IPTV VLAN configuration has been resolved with the latest DSA switch patches in the master branch. I still can't configure the VLANs using uci alone though. I have to additionally configure the bridge and ports as follows:
#Add IPTV VLAN PVID to bridge ports
ip link set br-iptv type bridge vlan_default_pvid 456
ip link set br-iptv type bridge vlan_filtering 1
The above did not work without the latest DSA patch.