Core OVN support was merged in OVN master and will be released with the next official version: https://github.com/ovn-org/ovn/commit/50f4ea011622a63c5ac0eec200c4176b1afe6fb6
OSP should either expose the ethtype field via API or just allow admin to configure it globally for the whole cluster via config file. The latter is obviously easier and probably the way to go since the setup is not as wide spread.
The following option should be set for localnet port that reflects a provider network:
<column name="options" key="ethtype">
Optional. VLAN EtherType field value for encapsulating VLAN
headers. Supported values: 802.11q (default), 802.11ad.
</column>