So I have an app where I want to insert a large number of rows into a table. The table is set up with one of the fields (let's call it UNIQUE) marked as a unique key (or something to that effect), and ...

Don't be afraid of using your chosen database's command-line client. I might as well say this up front: I don't like using GUI (aka non-command-line or graphical) tools with my databases. This is ...