Just use a molcajete. Either use it to grind them all the way or to finish off the grind after the coffee grinder. Unless you're making an absolutely huge batch it should only take a couple of minutes to finish up what you've ground. You could always then run it through however fine a screen you want to separate out any few remaining slivers. If you have one of the large Thai granite mortar and pestles I don't see why that wouldn't work also. I just use the molcajete for this because I figure the larger and rougher pestle allows me to more easily grind versus pound.
PS If you use a molcajete, make sure to season it and buy a traditional lava one versus some of the decorative models made partially of concrete or of soft rock.