MacBook Air with small SSD, BootCamp, Windows 7. Can I use an external hard disk for dual boot?

Question

I'd like to setup a dual boot onto my MacBookAir 128GB, with Microsoft Windows 7 Ultimate in BootCamp. However, I'm already close to the 128GB limit (games + CS suite + Eclipse + XCode + a lot of Web development tools). Therefore, it would be ideal for me if I could use the same HDD I will run my Time Machine saves on... when it's finally delivered! The question is therefore: is it reasonnable to create a dual partition on that and run Windows 7 from the first one?

Would MacBook Air tolerate launching Windows 7 from an external disk? Is there a risk (save for USB deconnection) that HDD might not be a working solution, as in too much lag? Could (eventually) a Thunderbolt external SSD be an acceptable workaround?