Coap - Unknown option

I‘m working on.a binding for the Shelly series of devices: Shelly Binding, but we can‘t register to get the COAP events.

Does someone has experience with the Californium framework? Shelly uses non-standard Coap options. When I register the observer no callback is performed. If I send a command I see ACKs in the log, but they report an unknown option 3332, which Shelly describes in their doc. I didn‘t found a way to register/inject custom options to the Californium framework so the observer can read them. Any help is appreciated.

AFAIK the Tradfri binding uses Californium. Maybe the developers of this binding can help.

check that, but doesn’t use non-standard options
I search more than once the Internet and didn’t found no solution
I contacted Shelly, they provided a code sample, but this is based on NodeJS

Does anyone has an idea?
Does someone know a Coap implementation, which could I use instead of Californium (and is maybe also more lightweight than Californium)

My isse: The Shelly API allows to set some event triggers, but they are very limited (mostly for relays/roller and the HT sensor). There are other events, which can’t be triggerd by a http callback, e.g. motion detection for the Sense, so I need to find a way to get Coap running.