While not 'intended', it is indeed possible on a portable. Exigent circumstances required me to re-role an aging MacBook Pro 2.16 Intel Core Duo to act as a server on a temporary basis.
It boots just fine from an external drive running Server 10.6.8. I also believe that installation (while not recommended) is not actually blocked.
Tip for the top: just be sure to pull the battery out of the laptop (assuming that's possible) to avoid turning your battery into a grossly distended, non-functional brick... they were not designed to be left on charge permanently!