I am using UPA Java and for ANSI Page payload we could use Msg.encodedDataBody(Buffer encodedDataBody) to encode pre-encoded data, however I am not sure if we can do the same for RWF data (e.g. FieldList)?
Yes, a pre-encoded field list can be added to an update message via Msg.encodedDataBody(Buffer encodedDataBody). You must then call Msg.encode(EncodeIterator iter) before writing the message to the wire.