Lol, if that's not jimmy rigging I don't know what is!
That sounds doable though and quite unique.
Few things to consider in my opinion:
1. Small enclosure to protect exterior HDD
2. Powering the external HDD will require more hardware and possibly make the netbook less mobile and more of a workstation, but that might not matter to you.
3. What about a few USB flash drives instead, they are quite affordable now and have good storage capacity, they can be easily removed to keep the form factor small.
Wow this opens a ton of new possibilities. Since its blistering cold maybe I can put my cpu outside my window and do some extreme overclocking!