Just a heads up that Proxmox have released version 4.0 and have added support for passing through serial numbers on hard-drives to the virtual machine.
It’s not exposed in the GUI but the trick is to open
/etc/pve/qemu-server/xxx.conf (where xxx is the vm id) in a text editor
Find the line relating to the HDD and add ,serial=Myserial to the end.
It should show in the GUI as a drive with a S/N if it accepted it, much easier than trying to pass custom arguments to KVM.
So for example one of my Data drives I pass through directly to the Vm looks like this in the config file
virtio0: /dev/disk/by-id/ata-WDC_WD20xxxxxxxxxxxxxxx,cache=writeback,backup=no,serial=WDC_WD20xxxxxxxxxxxxxxxx
Note: that the above is all on a single line, but the forum may have wrapped it.