Rick,
How 'bout little piles of ground up cigarette or cigar ash fixed in place with a little white glue a la static grass ? I know not everyone's a smoker (I'm not either) but a trip to any public building with outside ashtrays could do it, if you don't mind the weird looks as you dive into the ashtray - uh, maybe late at night might reduce the embarassment !
As an alternative, I think MIG makes pigments to replicate the ash - an article in TMMI magazine shows their use on a burned out T-55.
Hope this helps,
Brian
