Spoofing a game controller

Question

I'm trying to write something that will allow a user to use a keyboard/mouse combination on a games console. The approach I've thought of is to do the following:

Figure out how input maps to output in the console's controller. This is good old trial and error, basically connecting the controller via USB to a computer and figuring out how the controller responds to input using detailed USB logging.

Then, write a userland program that takes keyboard/mouse input and converts it to the controller's output.

So far so good. Now comes the complicated bit. I need to figure out a way to connect my computer to the console, while making the console think that what is actually being connected is a standard controller. For this, I will need a way to create a fake USB device, which communicates over a USB port to the console. This is the part that I have no idea whatsoever how to do.

I guess I'm looking for a USB spoofing library of some sort, but I'm not sure where to start looking for something like this. Python bindings for the library would be great, but I'm not fussy. Similarly, something that works on Mac or Linux would be perfect, but I could live with a Windows-only library in a pinch.