>  decodeStructure bitstream: (ExternalData fromHandle: aFrameBuffer
>     type: ExternalType byte asPointerType).
>
> This triggers the error: Only external addresses can be stored.
> So Is this just impossible? Do I have to write a real plugin?

If aFrameBuffer is a ByteArray (as I suspect) this will fail since the 
FFI will not allow you store addresses of objects living inside the 
Squeak object memory. Those are *movable* and storing the address for 
some C function call is meaningless - the Squeak garbage collector 
might have long moved this object away from this address and you'd 
start to overwrite random pieces of Squeak memory.

You need to create the byte array in external memory say:

    pointer := ExternalAddress allocate: aFrameBuffer size.
    decodeStructure bitstream:  (ExternalData
            fromHandle: pointer
            type: ExternalType byte asPointerType).