> So should I rewrite the SP as selectable? Any other ideas?

Can you check client-
java/src/test/org/firebirdsql/jdbc/TestFBCallableStatement.java ?

I just changed it a little bit and on my computer I'm able to

a) use CallableStatement with {call ...} syntax;
b) use PreparedStatement with SELECT * FROM ... (I had to add SUSPEND
to it, you are right, my first example was wrong.)

Best regards,
Roman Rokytskyy