QUESTION :
Windows 7 defaults to assigning the letter C: to the primary partition if you let windows 7 handle a raw volume. However, assuming there are no optical/floppy drives, would it be possible to use DiskPart, create the partitions as necessary, and then assign say, letter A:? Would Windows install correctly and be usable?
ANSWER :
No, Windows Vista and up always use C: as the system drive.
For example, if you have two partitions, one with XP and the second with 7 so that you can dual-boot XP and 7, you will see that when you boot into XP, it will still be on C: as before, but when you boot 7, it will also be on C:. What happens is that the system is mapped to C: while the partition that would have been C: will be mapped to something else.
The same is true if you dual-boot Vista and 7 or 7 and another copy of 7; each system will assign C: to its own boot-up partition and assign something else to the other partitions.
Further, A: and B: are special reserved letters that cannot be used as system drives, even in XP (though you can assign them to miscellaneous hard-drive partitions).