Hi, Martin.
[Passport-SAML doc](https://gluu.org/docs/ce/3.1.6/authn-guide/inbound-saml-passport/#altering-flow-behaviour) contains answer to this:
>In all cases, instead of adding a property via oxTrust, you have to manually add it for the proper provider in config file /etc/gluu/conf/passport-saml-config.json. Equivalently, if you need to alter properties of a custom script, apply those in the passport_saml script. Wait for about 1 minute for changes to take effect (no restarts needed).
Have you tried this?