2011+ 5.7 hemi mystery sensor boss?

I know the hole you're talking about. I was equally confused on my 06 Hemi when I saw that. It looks a lot like it had something to do with water jackets, which made me nervous leaving it open, but I couldn't find anything that told me it needed to be closed. I think a block heater probably sounds right though. Big enough hole to get a heating element in there.

As far as an oil temp sensor, that would be easy enough to add if you wanted one. On my 06 the temp sensor was installed where remote oil filter lines go, so I had to remove it. I'm not running a factory computer though, so I didn't need it. Have thought about adding it back in one of these days, but don't have enough channels to read it right now on my aftermarket computer setup.