Thanks to evilution.co.uk
I found out that the rear wiper from the new model Smart ForTwo (called the 451) fits the previous model's (called the 450 and in the USA is a "grey market" vehicle) rear wiper.
Here you can see the new model's wiper fitted to my 450 (the top wiper) and the old, non-smooth wiper (bottom wiper). What a difference. The new style wiper even wipes a bigger area than the old one.
Thanks to the gang over at Evilution and SMRT ROD for this.
Find more info about smooth rear wipers and all kinds of other things for all versions of Smart cars at evilution.co.uk/590