I took off my A2 flash suppressor earlier this week.  I wanted to check the crown I gave it.  The crown was full of carbon crude.  The standard flash suppressor has a small chamber before entering the slotted section.  I suspect that this traps the dirt and deposits it on the crown.  I checked another AR; it too had the same crude.  My two AR's without a flash suppressor have clean crowns; the crude must be blown away when the bullet exits the barrel.

Does anyone make, or has anyone experimented with making or modifying, a standard flash suppressor to alleviate carbon crude build-up?

It is perfectly normal for carbon to build up on the crown. Just leave it. It enventually breaks off on it's own. You have a better chance of damaging the crown than actually getting the carbon off trying to scrape it off.