Did you manage to find a solution for this? We’re working with AWS Cognito, and they default to their custom claims having the custom: prefix in the key, which Auth0 doesn’t allow due to invalid syntax, and I tried the same string in brackets thing that you pasted above.
We can use a Lambda to modify this at Cognito’s end, but asking all of our customers who might be using Cognito to do this might not be feasible.
This issue has been raised up internally and unfortunately regardless of the syntax used namespaced custom claims in OIDC attribute mapping have to comply with standard JS format, so they are currently not supported. It has already been added as an opportunity backlog, but if you feel like it I would encourage creating a Product Feedback so it can reach more visibility and possibly get more votes in order to get implemented.