Weatherproof Low Voltage transformer Hardwired
 

This is really a product question I guess. I am having trouble locating something (which may not exist).

I would like to hardwire a small 12V Transformer on to an existing "switched but empty" receptacle box on the (unprotected) exterior of my house. Is there such a product that is weatherproof, can be hardwired to a switched receptacle, mounted to the exterior wall at the receptacle, and will transform 110v to 12v AC? Oh, it should have a built in timer too....

yep.
Check your local big box home improvement center for replacement low voltage outdoor lighting transformers.

You think the local big box will have something that hardwires, n a weather proof manner, to a single gang box? All I have seen there is plug in models. I'll check again.

Yes, what you are looking for is included in most non-solar powered lawn/garden lighting sets (timer and all).

I have in the past found 12v transformers that would serve your purpose, but that's been a long while ago. You may find such in a full-blown electrical store. What I also have done from time to time is the mounting of a device in a steel or aluminum box which I sealed for weathertightness. I used gaskets, O-rings, sealing sprays, etc. to do so. I home-built an outdoor 12v power supply with an electric eye and relay which has been controlling garden lights for over 20 years without fail.