You can squeeze ERTO 406s on a Birdy with Stelvio tyres. However, the clearance is tight and the fold more awkward.
Steve Parry has done a frame mod for Birdys to allow 20" - see Spambait's earlier link to the Folding Society. I would also go to the Yahoo Birdy Group.
I am unsure (although almost certain) Maguras will not work with the bosses sufficiently for the blocks to touch the rim. As for a hub brake, I don't know, but it seems a more likely solution. Let me know how you go. The OLN at the rear is 135mm.
The Anniversary model has not quite sold out. I very nearly got a one from Germany, but my girlfriend was not swayed by 20" wheels, 18 gears and drop bars...
http://www.flickr.com/photo_zoom.gne...0426345&size=o