You could do this.
Close down iPhoto. Move your iPhoto Library folder to the external drive (pressing cmd - ie. the apple key - while dragging the folder to the external drive will move rather than copy).
Then on the folder on the external drive, right click (or ctrl-click), select "Make alias". This will create a file called "iPhoto Library.alias". Cmd-drag this back to Pictures, rename it by removing the .alias part of the name, relaunch iPhoto as normal. To iPhoto, it'll be as if nothing changed but it'll be reading from the external drive.
You would have issues trying to use iPhoto (it wouldn't have a valid library) if you don't have the external drive, but I believe it will create a new library called "iPhoto Library 1" in Pictures. When you travel without your external HDD, you could save the photos then load them into the main iPhoto library when you're back at home.