Adding a playlist to a cleared iTunes library, to be synced to half-full iPod my Apple-curious father has come up with a novel question - I was wondering if anyone had advice as how to solve?

Being fond of a certain type of music, he's gradually filled up his iPod with his music collection. As everything he owns has been transferred, he's blanked his iTunes account and no longer updates his iPod. All his music is in one place, for easy, portable, listening.

Problem is, now he has to add more - a playlist for an event he is DJ'ing at (no, really).

Adding this playlist to his blank iTunes account, and then syncing to the iPod would erase his music library stored on that iPod, would it not?

Sorry if there's an obvious answer to this - particularly if the answer is to sync the iPod back to iTunes to fill up his account, then add the extra tracks and move everything back to the iPod. That was my suggestion - wanted to double-check in case his beloved tracks are lost forever...
I never ever recommend using an iPod as the only form of storage for music. Inevitably that iPod will become lost, stolen or dead, and his music will be gone forever.

That said, there are two suggestions for what you asked. The first is to place the iPod into "manually manage" mode. This kbase article from Apple is a good guide that covers the different methods for different generations of products.

The second, recommended suggestion is similar to what you mention in the question. First move the music back to a computer. You will need special software to do this, such as the cross-platform CopyToy. Once the music is back in iTunes on the computer, create this new list and sync ALL of the music over. The iPod will be wiped of all of its music and replaced with the same songs, plus the new playlist. And while he's at it, have him backup his iTunes Library so he doesn't lose his music.