If you're doing an all grain batch, why not just "cap the mash" with the dark grains right before you sparge? I do this all the time, especially for Schwarzbiers when I don't want Carafa/Chocolate to go through the decoction. Works great with zero astringency and subtle roast flavors.
For the cold steep, it would be a good idea to plug in your water and grain quantities to a water calculator to be sure your pH isn't too low. Also check the pH with strips or a meter just to be safe.
I've never cold steeped but would love to learn more behind your method/reasoning.
Cheers!