If I understand correctly, it can't do this at the moment,

ref:
http://www.nabble.com/Using-manager-to-deploy-with-path%3D%22-123-456-789%22-t4200949.html

Would be good if it could.

I'm thinking that an administrator would be able to define a location outside of
appBase where war's for such apps could be expanded, e.g.
<catalina_base>/outsideAppBase 

The manager on recognising such a contex-path could then expand the war there
(in a subdir according to the context-path specified, and create an appropriate
context xml file to go into <catalina-base>/conf/<Engine>/<host>/

Interested to hear what you think, and if you forsee any problems with this
approach.