if you've ruined the bootloader so bad that it can't even boot into DFU mode then you're pretty much "bricked"
The bootloader is actually contained in the nand part of the memory partition, and the N900 has 32 gb eMMC and 256 mb NAND non-removable storage in this partition but however if your bootloader is damaged (and i have never known of an incident like this on the N900) it is however flashable but not via the flasher programme used for the firmware, so all would not be lost , ie not bricked.
I think very very unlikely the bootloader can actually be damaged short of being subject to radiation, but please prove me wrong here.
I think very very unlikely the bootloader can actually be damaged short of being subject to radiation, but please prove me wrong here.