This is a very interesting article.
http://www.terra-char.com/uploads/2/3/7/9/23790961/poultryfarming-acresusa.pdf
I've been interested in adding charcoal to their diet for a while but no one has really been posting about it...until your post!
How are you calculating your 5%? Sorry, I'm terrible with mathematics! My bag of Biochar for gardening purposes says 1 cup Biochar to a 2 gallon container of dirt. FYI, I originally bought this for transplanting my dahlias.
Interesting. I noticed a difference in poo right away after I put the charcoal in their dust mix and they at it. The poo was much drier and definitely black. Charcoal also aides digestion.
I completely forgot that I had this stuff in my cupboard!
What is your purpose for adding the biochar to their food?
I have regular charcoal (from clean burning wood only) in my chickens dust bath mixture. I notice they go through the dust bath and eat the charcoal bits out.