Using COM (or DLLs) to make data-access routines customizable by (smart) end-users.

Overview:
I have built an inventory analysis algortihm that must be customized by the end user to integrate with legacy data systems. I want to architect the application in a way that simplifies the integration.

Details:
The demo version of my application uses global functions like this one:

int GetInventory(COleDateTime sampleTime, int resourceID)
{
      // Use input data to seed random number generator.
      srand(resourceId + sampleTime.GetDay());
      // Generate random data for demo implementation.
      return (rand()%20);
}

Instead of retrieving data from a database or other source the demo version just makes the data using a random-number generator. When integrated in a real business environment, the logic should be changed to retrieve the data from a real database. Therefore, any end-user my application would want to customize the definition of this function to pull data out of their own database and possibly do related computations.

To facilitate the customization, I might give the end-user the entire source code for my application and let them alter the implementation of GetInventory() and recompile.

That's a bit sloppy, however, and I would like to know a more professional approach to acheiving the same purpose. I think COM can help, but I need guidance.

Work In Progress:

I guess I could define a simple COM object "IMyDataAccessRoutines" that my application uses and change the implementation of GetInventory to:

int GetInventory(COleDateTime sampleTime, int resourceID)
{
      int response(0);
      IMyDataAccessRoutines* pMyDataAccessRoutines;
      HRESULT hr = CoCreateInstance(???,???,(void**)pMyDataAccessRoutines);
      if(SUCCEEDED(hr)) {
            pMyDataAccessRoutines->GetInventory(sampleTime, resourceId, response);
            pMyDataAccessRoutines->Release();
      }
      return response;
}

If I put the rand calls in the COM object implementation and deliver the COM object as a DLL, then the end-user could customize the application by building his own version of this COM object and replacing the DLL on his system.

Question:
How do I make this work correctly?
COM is so confusing that I am sure I will make some mistakes if I don't get a little help.

What I would ideally like to see is complete instructions for dealing with this simple example.
The solution should say how to:
1) Implement the global function GetInventory()
2) Build the default COM object or other DLL.
3) Build the end-user COM object or other DLL.
4) Switch the application at run-time back and forth between using the orignal or end-user version of the DLL.

The example is simple and clear, so I am really hoping for a clear explanation for this particular case.

Cheers,
Diana