On a USB flash drive: yes.
On an iPod nano: no.
You're basically asking whether you can make an external bootable disk. The answer to that question is basically, "Yes".
The reason an iPod nano can't be made bootable is because you can't usually make it look like an external disk to the Mac's firmware. Anything that
does look like an external disk, whether it's USB or Firewire, whether it's flash memory or spinning circles of aluminum, can be made bootable if it has sufficient storage capacity and is the correct partition-map format.
Since you already have a bootable disk, the simplest is to clone that to an external disk. Two commonly used tools for that are Carbon Copy Cloner and SuperDuper. You can google their names to find downloads for them.
You must also partition the external disk correctly, and create a volume with the correct file-system format.
See here:
http://support.apple.com/kb/ht5911
That article is about installing a virgin copy of OS X to an external drive. So do the steps under the heading
Erase and reformat the storage device, but then stop when it discusses using the App Store to re-download the OS installer. That's the point where you'd switch to using Carbon Copy Cloner or SuperDuper to make an exact replica of whatever bootable drive you have.
I have several external disks I connect with Firewire. I rotate bootable copies of my internal disk between them. I use SuperDuper at this time, but I've also used Carbon Copy Cloner, and other tools.
I've also used an SD-card inserted into the MacBook's SD-card slot, and that also boots. It's not speedy, but it works.
The key thing is to have the correct partition type (GUID Partition Table for Intel-based CPUs), and to have a large enough partition for the copying to succeed.
Finally, you should test that the resulting duplicate is actually bootable by booting from it at least twice before doing anything that might alter or damage the internal disk drive it's copied from.
