Advanced externalizers not supported by AS7 subsystem

For our schematic classes, we use ISPN advanced externalizers for marshaling & unmarshaling. Those externalizers get set in the global ISPN configuration (for a local environment).

However, when running in AS7, those should be registered with the ISPN AS7 subsystem. Currently it seems that this isn't supported by ISPN (see forum ref).

The current workaround is to use user-friendly (non-advanced) externalizers or to make the Schematic classes implement Serializable.