Yep - the armature is a standard in the US - and it's called a 2" receiver - it's possible that the US FFRR's are fitted with 50mm receivers and customers are forced to buy accessories from main dealers - but anything and everything else in the US market is designed for a 2" square hole.

It's coincidentally the same size as the balls - the EU use 50mm - the US have several sizes, 2" being the most common, but not the only one in wide use.

I used to have a Jeep Wrangler with a 2" receiver - some of the bits would fit the FFRR, but most wouldn't.

I've already got a Thule towball mounted one, but I find it a bit of a chore. I took the two bikes to our holiday home last week and I had nowhere to store the rack afterwards. I ended up putting it in the boot which then takes up room. Also, I have to tilt it to open the rear hatch which is a pita with two bikes on.

if you use some of this, 50mm steel box in 5mm thickness, 1 metre long at £18 http://www.ebay.co.uk/itm/Steel-Box-Sectio...259a6cb57b and fabricate any 'already made up' bike holder on to the steel, you are almost done.
if you use the 'bike frame crossbar' hanging type of bike mount, you sometimes find, that other guest bikes have odd shaped frames etc. however, if you use a wheel mounted rack like one above, you can fix 'any' bike to it.
you could use roof rail bike mounts, welded in parrallel to the hitch mount method, to fix your bikes to etc. -and hang a trailer board to stop the rear lights/reg plate being obscured etc.