Yes, I know, they only ship to The Netherlands and Belgium but any Riese & Müller dealer (even if they sell only R&M ebikes and do not sell the Birdy) can order these parts for you and any Birdy spare part if you tell them what part you need.

Your solution that use a Brompton chain tensionner on a Birdy is very nice. Its only drawback is that it works only on a bike without rear derailleur.