Alternatively, it might be possible to (ab)use the delegate support in RDNZL to get what you need - simply define the delegate signatures you require in an interface class on the C# side, then use RDNZL to create an interface object and attach lisp closures (I do something like this to implement LISP callbacks for a somewhat misdesigned .NET library).

Ok, I create a shared .NET DLL that defines the interface and all needed classes. I can deliver a "C DLL" with LispWorks that is able to instantiate the types and interface as well as register callbacks with these types that it has instantiated.

The fundamental issue that I cannot work around here is that there is no clear way to pass along the previously instantiated from the .NET application through to any functions exposed by the Lisp DLL.