> Is it possible to make a field unique without specifying an index on it?

Do you want to avoid _specifying_ index of _creating_ index at all ?
You can create unique _constraint_, but it will create index internally anyway.

Do not think about using triggers - it is not multiuser safe,
and without index it will be slow on large tables.

Ivan
http://www.volny.cz/iprenosil/interbase