It's tied to the FubuMVC.Core but can be used independent of any web host. However, you can have an embedded Katana/OWIN host and expose interesting Diagnostics and Instrumentation into your Windows Services (we're already using this). I would say that friction aside, having been a contributor to FubuMVC for a while now, I started to see benefits that could be very useful in a messaging system and I didn't see that being very easy given RSB's "close to the metal" and opinionated approach. Not to mention that the original author wouldn't recommend using it anymore ;) Here is a few things that we consider beneficial.
Multi-transport in a single process i.e. running both zeromq, and rabbitmq if needed
Collapsing a distributed system into memory for easy fast integration testing
Scenario based testing that can assert what your system sent and received (all correlated w/ cause and effect)
Leveraging conventions to minimize coupling to the messaging framework
Poco handlers (via OOTB conventions)
BehaviorChains for your handlers.
Pluggable / Extendable Serialization
Rich modularity via Bottles
Pluggable Exception behavior (one of the most request features for RSB)