I like the guard nodes idea ("gateway" or "proxy" nodes?). Maybe introduce a gateway node type, and add a way to pass traffic through a particular gateway (with a second tor-like encryption layer) defined using an extension to the current flow rules language. That would collapse all the traffic behind the gateway into a single identity (the gateway's), and probably be general enough to support all kinds of fancy features like onion-routing, monitored link balancing, etc.