That wouldn't help. The problem is the trapped debris in the substrate is exposed to the water the discus are living in. Since the sump is the same water it wouldn't help. Now after everyone has beat you down on the substrate let me provide you some hope.
http://forum.simplydiscus.com/showth...sh-anniversary!!
Now a few differences, those fish are probably 4-6 months older than yours when introduced to a tank with substrate these few months are the most important time. But it shows you can grow out sub adults into very nice fish but it is a LOT of work. With 2-3" fish it simply is not possible. If you can I would suggest a BB tank AT LEAST 3-4 months if you must rush things. Then again if you are OK with having not perfectly huge fish and you are meticulous in your cleaning you can keep them healthy and growing well enough in your tank. Take all of our postings as a warning of how extremely difficult it can be. But it can be done.