Sure, building that kernel is possible in Linux VM but having a simple plugin for building that same kernel would be even better. Sadly, I have no experience on compiling in Linux... Just like I can manually do the same thing as VFIO-PCI plugin but using that plugin is simply faster, easier and much more noob friendly.
What is the main reason for Unraid having to be as light as possible? I doubt anyone is running out of space on their USB sticks... But if so, having two different versions of Unraid could help as well. The normal... And the other one with tons of extra drivers.