well, the 12v device won't run directly in line because the system has at most 9v power.
What I would use is something like http://search.digikey.com/us/en/product ... -ND/678187
they are expensive but I have several that I have scavenged out of various broken machines. So what you are looking for is something with an operating voltage with a low end below 9. And the load voltage higher than what ever you want to plug in. These ones will let you turn on your coffee pot, so they are overkill I just use them for various automation ideas.
You mentioned a diode, were you trying to setup a protection diode for the load circuit?