How is the fabric sealed?

According to iFixit's teardown of the Smart Cover, it doesn't seem feasible to attempt doing so. The design does not lend to being modified by any means and will likely destroy the Smart Cover should you try.

I'm not saying don't do it...I'm just saying that if you do:

1) Take the proper precautions against flying parts (see teardown).

2) Document what you do, with pictures, so you can post a guide here for others to reference.

3) Make sure you are willing to part with a perfectly functional Smart Cover to do this (or at least have the funds to replace it).

While the inquiry isn't strictly repair-related, I can see its utility in case a Smart Cover ends up damaged in some way (acid spill, cigarette burns, etc).

I hope this helps in some way...tho I'm sure it's not quite what you were hoping to hear.