Join Date: Jun 2009
Location: Somerville, MA
Bikes: '14 Cannondale CAADX Disc 6, '11 Trek Earl, '09 Xootr Swift, '04 Giant OCR1, '96 Lightning P-38 (for sale)
Like Thor said, there's the Twofish one that lets you mount a water bottle cage to about any tube (seatpost or steering riser). If Thor sells it (I can't remember) you should order it from him, but otherwise you should be able to order it from your LBS or about any online place that sells things from the QBP catalog (aebike, bikeman, etc). I got one for my Swift recently and haven't attached it yet, but I am using Twofish's lock holder and it's perfectly good (better than the Kryptonite one).
Profile Design makes a thingie that clamps to the seatpost (should work with a steering riser too) and holds two water bottles. And I think Minoura makes a water bottle cage holder that attaches to the saddle rails.
If the water bottle cage interferes with the fold, I think Rixen Kaul/KlickFix makes something that allows the cage to quick-release, leaving only a very thin adapter attached to the bolts. (You'd still need something like the Twofish or Minoura holder to provide the bolts, though.) And I think the Twofish one uses rubber and velcro, so it should be fairly quick to detach even by itself.
How's that for an overly detailed answer? :-)