Could you use a centre lock ring that has the teeth on the outside of its diameter - the one secured with a BB spanner or socket? I bought one for a DT Swiss hub:
Usually they tend to supply with that lockring when it come to thru-axles bike, reckon you're right that they didn't realised that they need to add a new lockring to use the Shimano rotor instead of the one that come with it.
Could you use a centre lock ring that has the teeth on the outside of its diameter - the one secured with a BB spanner or socket? I bought one for a DT Swiss hub: