Does anyone have any recommendations for a rear-mounted (rather than a roof-mount) bike rack for the xB?

funny you should ask...while i was looking at the new wheels PIAA has introduced, i noticed this -- PIAA's minivan 1 box ladder
http://www.piaa.co.jp/piaa/02_product/carrier/ladder/img/ladder_photo01.jpg
more info here: http://www.piaa.co.jp/piaa/02_product/carrier/ladder/index.html

Any universal bike rack should work, from a bike store or Walmart. I have one that I bought years ago that is adjustable. Fits on back door perfect. :o