Maybe the suggestion of drying them out more first by popping them in the oven would work for you.
I made some of Maddy's Cheese and Bacon rolls yesterday morning (yummy btw) and put half a dozen cinnamon quills into the oven after I turned if off. Left them there till this morning and then milled them for 1 minute, speed 10. It worked really well - much better than previously where I had lots of bigger bits left.
Also, the Indian Cookbook says to grind your quills (at least 2 but no more than will cover the blade component) for 1 min, sp 10 and then to lay out and allow to dry out overnight. Then return to the bowl and mill again for 2 minutes, speed 10. I haven't tried this way yet but perhaps that will work for you?
Cheers,
Kerryn