I am unaware of a counter that shows if the deswizzler is active but if somone finds one it would be trivial to add.
The only way I know to get this info is using the wafl scan status command. So you could write a small script that would SSH to the filer, grab this command output, parse it, and submit graphite metrics with a boolean value of running or not for each volume.
I don't plan to code this myself into Harvest (too much collection overhead for not enough demand), but I was thinking to include an 'extras' folder in Harvest for exactly this kind of thing so if you create it please share!
Cheers, Chris Madden
Storage Architect, NetApp EMEA (and author of Harvest)