Windows cannot modify the drve letter of system volume or boot volume and if I go to another windows (installed on different partition2), it is meaning less because Disk Management there, will save settings on it's own windows partition2 and not on partition1).
I suggest you use Disk Management and/or MountVol.exe from within the the OS whose drive letters you wish to change. It seems that you are interested in choosing the drive letter for the boot volume before ever booting the OS. Is that correct?
Not sure how imDisk is related with system volume's drive letter.
It isn't. By "contrast," I meant that you can observe how Firadisk and ImDisk differ. The former provides disks, the latter provides single-partition "disks," which are essentially partitions. Since drive letters are associated with volumes (partitions, in this case), that is why it can make sense for ImDisk to choose drive letters, but not Firadisk.
It's not impossible, but it seems out-of-scope for a disk driver to work with partitions. There are different partitioning schemes, for example: MBR, GPT, APM, etc. Should Firadisk have to know about each of those? Should an Intel SATA driver have to know about each of those? Should a vanilla IDE driver have to know about each of those? That is why partitions are provided by a higher-level driver.
At the time of fresh install, it is not
Did you mean to type "is it not"?
possible for "a new feature" to calculate existing number of drivers, map some memory, and reserve all remaining drive letters till %custom% specified drive letter in menu.lst so Windows mounts that exceptional drive letter? After installation all reserves are removed (by choosing normal menu item from menu.lst?)
Disk versus partition! Drive letters are for partitions, not disks! Why should a disk driver know anything about partitions or manipulate which drive letters are assigned to partitions? A disk might not even have any partitions on it, or be entirely one partition (like a "superfloppy").
You could potentially partition a disk image as per your option 03
in order to control drive letter assignments before installing the OS. However, my impression was that Windows NT gives primary partitions drive letters before logical partitions.
If you want a feature that manipulates drive letter assignments, it ought to co-operate with the normal
components responsible for assigning drive letters, and it shouldn't be a feature for a SCSI disk driver, in my opinion.