Currently Jaybird will always use the default system encoding of Java for converting between strings and bytes when no connection characterset or NONE is used for connecting to Firebird.
It may be a good idea to change this to first try to use the characterset of the database itself. If that is NONE as well, then fallback to the default system encoding. The connection characterset would remain NONE, but the encoding used in FBStringField would change to match the database - if possible.
Although this sounds good in theory, it will need to be tested to ensure it works OK and does not cause compatibility problems, and if it is easy to efficiently retrieve the database characterset.