TRF member rocket999 provided a link to the following interesting article: Direct Non-electronic Staging From Reloadable Composite Motors To Black Powder Motors, Jay Calvert, NAR # 71767 C Division, Naram-46, Great Meadow, The Plains, Virginia, August 2004.
Jay studied several potential methods and found that you could reliably ignite upper stage BP motors from a booster stage APCP motors using staging igniters. He used both the pyrogen covered ends from Copperheads as well as home dipped ones. The main issue appears to be finding a composite motor with an appropriately short delay. You obviously want the sustainer to be pointed upward when it ignites! Unfortunately, in most cases the standard Aerotech -4 second delay is too long for most applications (especially if you get a 'bonus' delay). A powerful booster and a light sustainer may be workable. Drilling AT delays below 4 seconds is not recommended and would be uncertified. Jay also points out that you can get down to a -3 second delay from CTI motors.