A friend on one of the mailing lists I read pointed me to this web page which contains the following description of a textile find from grave 619 at Birka:
An early 9th century woman’s grave (619) yielded a beaver fur lined with hempcloth, and a mid-10th century woman’s grave (837) yielded the remains of a caftan lined with hempcloth. The texture of these two finds were similar in fineness to high quality linen.
The Historiska Museet contains an indexed database of the Birka finds, and I soon located the finds associated with grave 619 here. I assume that the find referred to as a "komposit fragment" is the one I'm looking for. However, I can't decipher much else about the entry, and I'm having trouble figuring out where it was found in the grave, or anything that would give a clue as to what type of item it might have been part of.
If any of my readers know anything more about the beaver fur lined with "hempcloth", please comment! I'd really appreciate it.