That's how I fitted a lap belt in the cosworth. Used the outer mounting points.
Mbenznl has cls seats in his 211 so it shouldn't be too difficult to go the other way round.
There are conversion kits on ebay for vehicles like the x6. Might be worth dropping them a bell and see what they can offer you.
I think that's the answer for the OP then : find a 211 in a breakers ( with the same colour interior as his own car ) and try the back seat to see if it will fit . Assuming it does , he can store all the original bits from his own car and have the option to put it back if selling the car later .
A lap belt from pretty much any saloon should fit the existing mount points without difficulty .
I must say I like individual rear seats and still keep my eyes open for a 560SEL breaking with that option , which I always fancied retrofitting them to my 500SEL , currently with the electric reclining rear bench . Sadly , not so many 126's to be found breaking these days .