Thanks @Vegz78 for the words of support. Yes, your issue is the same one I’m having and it would be great to be able to support more custom user-made controllers. I have limited technical knowledge in understanding what the underlying issue is but would be happy to make changes on the custom controller end if there was guidance on what needs to be done.
As for the build, I took up @UnsignedArduino 's idea (thanks for that!) and tweaked the original firmware to send keyboard events in relation to analog readings from the joystick and it worked! Here is the modified file in a Github Gist. Again, this is a very rudimentary patch. I sent the code along to the MMC engineers and perhaps if they have time will be able to have a more elegant solution.
And yes, I too want to thank all the wonderful people at MCA for making such a great platform and being so incredibly dedicated and responsive! You all are amazing!