Ehh no, drivers do not do that. Kernel drivers live in kernel memory space which is entirely separate from user memory space, and it's the kernel and wddm that does all that segment mapping and management. Drivers would only be responsible for the physical memory itself, and that is not linear. It maintains it's own internal table of physical addresses to memory buses.Drivers can most definitely can define their own address space and map to physical memory regions in a driver.
Having said that, locking VRAM size is trivial by just not having the driver report it to the OS or just flatly refusing to address physical ranges beyond some value. No need to make unstable drivers and break modern Operating Systems.
In any case the idea is kinda... not smart. Then chip already has a defined bus width that determines memory size and speed. Sticking 1TB of VRAM won't change anything as compute and IO are bigger limiter in such an entry level GPU. Aka there is a reason the 5090 has a 512 bit memory bus.