Call Assembly Business Rule from another Workspace in a Data Management Step

The problem here is the nature of the code you're trying to call. Data Management steps can either call:

• A traditional Extender, which is a class that must have a Main function accepting 4 specific parameters; or
• a Service Factory, which in turn will provide a Data Management Service with a specific method to call.

From the WSMU reference you see in the original step, we can deduce that OFC is doing the latter: it's calling a Service Factory set in the properties of the current Maintenance Unit (-MU) in the current Workspace (WS-). You can try pointing to that factory with "Workspace.OFC.RCM.WSMU" (or something like that, I don't remember the details of RCM maintenance units), but there is no guarantee it will work - because that factory likely assumes to be called in specific ways as part of OFC operations only.

If that doesn't work, you will have to write your own Factory which somehow invokes that code. If you examine the code of the original factory, you will also find that it's also doing something a bit funky, implementing the service through custom classes that implement a parent event-like class. You might be able to copy all that stuff too, but it will require careful examination of the code to figure out what you need and what you don't need.

In short, it's probably not as simple as just pointing at that code...