So the S211 factory towbar was never offered in the USA, but I'll be darned if that is going to let me take a Mercedes to U-Haul for an aftermarket installation or try to do something myself involving drilling into the car.
In case you have this same inkling, the two important part numbers are:
2113100104 - Tow bar
2115450832 - Control unit
The control unit was easy enough to find on eBay shipping out of Germany (though a surprising number of other MB Parts do not recognize the number at all), however the tow bar proved to be more of a challenge. I could find a couple used units on eBay.de and one in Australia, but nobody who had them was willing to ship to the USA. Mercedes-Benz blocks shipments of parts to the USA if they were not originally offered in the USA market. Many sites list the part as available along with a price, but if you try to order or call and ask first, they'll say they can't ship it to the USA. I was able to determine there were three new ones in Germany, but no idea how to get them. I briefly considered having relatives in the UK order it for me, then ship it here... Even the used tow bars are rather expensive though, and since I knew the new part existed still I wanted to find a way to get it.
Anyways, in case anybody else is on this same Unicorn Quest, I finally found a place to buy the tow bar new from, shipped from Germany for a rather amusing figure of $13 shipping cost. Before you get your hopes up too much though, the price of the part is $1343. Still undaunted? There should be two more sitting in Germany waiting, and you can order it via this website:
https://is.gd/Y4hsEx (short URL used because PeachParts is blocking ECS Tuning URL's for some reason)
Hope this helps someone... If anyone has added a factory tow bar to an S211, I'd love to hear about any other concerns I should think about!