Would it be possible to create a custom virtual Adaxes Property
Unfortunately, it is not possible. The number of Adaxes custom attributes is fixed and cannot be changed.
setup a Business Rule to copy the internal MsExchActiveSyncEnabled value to it? And then display the custom virtual Adaxes property on the portal?
It is generally not possible to pull the value of the MsExchActiveSyncEnabled property. As we mentioned it is only used on the Adaxes backend. As an option, it is possible to have a scheduled task set a custom Boolean attribute of a user to true in case the user has the Exchange ActiveSync feature enabled. Also, there will be a business rule triggering After modifying Exchange properties of a user that will update the attribute according to the feature state. Will the approach meet your needs? If not, please, describe the desired behaviour in all the possible details with live examples.