If high-resolution timestamps for each keystroke aren't too much of a burden it seems like this would be straightforward to implement, given an algorithm like BioPassword's.
It looks like BioPassword works best on short, often repeated phrases like login names and passwords. I'm not sure how it would work for larger, unpracticed blocks of text. Etherpad would also need to store information about the timing of when each key is pressed, and based on the demo it doesn't look like they're doing that.
If high-resolution timestamps for each keystroke aren't too much of a burden it seems like this would be straightforward to implement, given an algorithm like BioPassword's.