I need to create a driver, which will behave similar to software RAID. E.g. driver will need to communicate to multiple physical disks (or maybe even network resources), and shall look like a disk to the OS.
So two main questions are:
1) Are EFI drivers recognized and supported by Windows, MacOS X and Linux? E.g. can these systems use EFI disk drivers, and ?
2) Is it possible in theory to write such a driver for EFI? My primary concern is possibility of accessing other EFI disk drivers from your own virtual disk driver.
I only have time for a quick-ish reply, forgive me for brevity!
To my knowledge, only the bootloaders for these OSs use the UEFI driver stack for loading the native OS kernel and drivers. Once ExitBootServices() is called by the bootloader, most of the drivers are unloaded, and (according to the spec) no calls to handle-based drivers may happen after this, meaning no disk drivers. Like a traditional bootloader, a UEFI bootloader is only using the basic drivers long enough to load the OS's native drivers. You can also use these drivers in the preboot environment if you'd like (although it sounds like you don't!).
TL;DR No, these systems can't use UEFI drivers other than for loading the OS.
You should definitely be able to layer your UEFI driver on top of the existing stack. It might be a little tricky if you haven't worked with UEFI before, but conceptually the system is very modular. There appear to be a number of resources on the Internet to help you out, and there is always Beyond BIOS: Developing with the Unified Extensible Firmware Interface by Vincent Zimmer.
As far as testing goes, you can use one of the simulators provided in Intel's EDKII (if you're on Windows, you should probably use the Nt32 project, it works well with Visual Studio).
TL;DR Yes, you can write this driver, but it will only work for bootloaders and applications in the pre-boot environment.