I wouldn't do any of those.
Leave your catalog file right where it is. Don't move it.
With the new external HD plugged in, open LR and use Lightroom to move the desired photos from the Z drive to the external HD. You do this via drag and drop in the Folder Panel. (Alternatively, move the photos using your operating system and then re-link in LR)
Go on your trip, take all the photos you want, import them into LR and have the photos copied from the camera card to the external HD.
When you get home, reverse the move. WIthin LR, move the photos from the external HD to the Z: drive, using drag and drop in the Folder Panel. (Alternatively, move all the photos in your operating system and re-link in LR)
And please, before you do any of this, make sure you have backups of all of your photos and backup of your catalog file. Don't skip this step. I don't want to see you in here after the trip saying something went wrong and you've lost all of your photos and you have no backup.
I thought of that. Guess I was looking for a more "elegant" way to do this using the catalog export/import Lr capabilities (havn't tried that yet). But, I think your right. The way you suggest is the most straight forward and hard to mess up.
And don't worry. I'll make a copy of everything I plan to move before I move.
"Elegant" means different things to different people. To me it means simple and straightforward. To other people it means making a clever use of a feature in a way that most people would never think of. To tell you the truth, what I have suggested fits my criteria of "simple and straightforward", and I wouldn't ever look for anything else.