2004 to 2007, Anand Babu has been working some on this project. It is still in rather early stages. It's meant to become an extension/complement to rpctrace.
A C language library for interposing ports of a Hurd task running on top of GNU Mach micro-kernel. Using this library, it would be possible to implement a trace/replay system, RPC debugger, sandbox, etc.
On top of that, a debugger was planned:
A RPC level debugger with useful command set to analyze/manipulate a task at run time. For example, the user will be able to set RPC break points, manipulate port rights and data, trace and replay a task.