I had never heard of late mash additions of specialty grains before and now I’ve seen it twice today.
How common is this, what is the benefit, and is there a trick to it?
I do this often for beers with a large portion of dark malts. I cold steep the dark malts, and then add them 5-10 minutes before the vorlauf. You get much of the color and flavor, but may want to up the amount by half or even double. The advantage is that you don’t get the burnt/acrid taste that you can get if you mash the whole time. My wife likes beers that don’t taste like an ashtray, so that is the way we do it.
This would also work with cara/crystal malts, since not much happens in the mash with these. It would free up some space if you were doing a really big beer. Mash the base malts, steep the crystal, and add the crystal when you are vorlaufing if you have room, or once there is room in the mash tun as you sparge.
I have not seen the other posts you speak of…, but I have done than for color additions a few times.
However, my process is to grind up my dark grain, when I do not want a lot of flavor from it… just color… into more of a powder and then sprinkle it on top of the mash before sparging. That way there is less extraction of the flavor. I used this a lot with chocolate and Carafa Special (huskless Carafa). Useing the Carafa Special helps a lot to reduce some of the flavors I don’t want, but provide the color and some roastyness as well.
More or less the theory is if you soak the burnt husks of the dark grains for a long period of time, the more tannins from the husks are released into the mash. For some beers that is a good thing… in others it is less so.
I’ve done what Jeff has done with good results. It really comes down to the matter of pH. You can mash the dark grains in there for the full time, but the addition of gypsum or other pH lowering compound is almost imperative to keep the “ashtray” out of the flavor profile. 
By doing the dark grains late, it minimizes the chance of the acrid flavors while still giving the character and color of the dark malts.
In the current issue of Zymurgy (September/October 2010) Drew Beechum wrote and article entitled “More Beer From Your Brew Day” which mentioned the late addition of specialty grains to your mash tun as a method of obtaining two beers from two separate runnings of your mash tun - a pseudo parti-gyle.
[quote]Instead of using the blended runnings, adding more grain can bump the second beer’s character. “Capping” [sic] the mash entails a small addition (0.5-2 lbs) of crushed malt added and soaked for a few minutes before the sparge is resumed. Common additions include color malts alike crystals and roasts to boost SRM and malt perception.
[/quote]
- Drew Beechum, “More Beer From Your Brew Day”
Also, in 1999 George Fix on HBD was quoted talking about the late addition of the liquid from separately cold-steeped specialty malts to the end of the boil. He is quoted as having said this method was designed to, “maximize the extraction of desirable melanoidins, and at the same time minimize the extraction of undesirable ones”:
http://hbd.org/clubs/cascade/public_html/brewing/index.html
I hate it when I get logged out after typing a post and have to re-type it. It’s bologna - BOLOGNA!
Don’t you mean a pH raising compound like chalk? Dark grains are acidic, and I thought the acidity was what extracted tannins from the husk.
Great feedback guys. Thanks. I had no idea folks were doing this.
Actually, that was the context where I originally read about this topic. Some people’s mash was getting too low pH — too acidic — with the addition of dark grains for a stout. Rather than adding anything, the suggestion was to just mash the base malt and steep the specialty grains separately, or add them to the end of the mash. Fascinating! 
Yes indeed that’s what I meant! :-[ Getting old sucks . . and posting after drinking doesn’t help!
You can also do some searching of the web if you want to find out some more.
I remember that Mary Ann Gruber of Briess was the first to talk of it. Here it is covered by the late George Fix.
http://hbd.org/clubs/cascade/public_html/brewing/index.html
Brushvalley-
Following up on the thread I started about water additions for an Oatmeal Stout-I contacted a local brewer here in Bend (Silver Moon Brewing). In addition to the recommended water additions he also suggested adding the dark malts to the last 15 mins of the mash.
the quote: “You can add your dark grain at 45 minutes into a 60 minute mash to smooth the harsh dark malt flavors.”
what hopfeundmalz said.
I especially do this for beers where I am using carafa spezial and only want color contribution.