My horse has such a dusty coat, there is dust right at the root of his hairs! I gave him a bath a couple of weeks ago and he was dust free for a few days but it came back so quickly.
You could try Carr&Day&Martin Canter Coat Shine. Groom him, then spray this all over him, and it should stop dust, grass/stable stains etc :) I use it on my piebald, and it keeps her sooo clean
You can also try Absorbine's : ShowSheen.. It's really used to shine their mane and tail; but it works pretty well with keeping the dust off. Just be careful using it on their back where you put your saddle and pad, it tends to make them a bit slippery!
Could you also use the Sante Fe (orange bottle) to do the same? My boyfriend's horse always surprises us with the degree of dirt on him, and its so hard to really make a difference in his coat because the dirt is like down by his skin. We do curry him, but ti just doesnt seem to be doing a good enough job.