hmmm... seems to me that I don't completely understand your problem...

Why cannot you:

a) use UNICODE_FSS everywhere?
b) use NONE everywhere?

(here "everywhere" means in servlet container, jdbc connection,
database default character set, char and varchar column definitions)

Well... I have not too much experience with unicode in Java, but the
problem you're describing seems to be either in JDK (that is not able
to translate chars between charsets correctly) or in
InterBase/Firebird that has no support for \u0151 or \u00f5
characters.

My personal opinion is: we should not perform any conversion in the
driver that goes behind the Java and InterBase/Firebird conversions.
If there are bugs in JDK or DBMS they should be fixed there.

I'll try to read your letter again tomorrow (it's 23:00 right now and
I'm not in good condition).

Best regards,
Roman Rokytskyy