I used to feed out of the cage as well, because I heard it tames the snake down. But I had a separate enclosure that I did this in, so I didnt have to handle them right away and stress them.
I dont know if boas are normally a bit more subdued feeders than burms or retics, but I can tell you that messing with a 9' burm in feeding mode, can be quite a scary incident. My burms and retics, even though I feed frozen, change dramatically when its time to feed. It truly is a jekyl and hyde trasition. I have a 13 ft female whom I fed yesterday almost take a chunk out of me. This is a snake that normally has no quams whatsoever about being touched inside her enclosure. But I moved a bit too quickly after dropping the rabbit and was about 2" away from her open mouthfull of teeth.
Ive never owned a boa, but I can tell you that you will probably want to start feeding in the cage with your burm. if you handle your snake regularly, and make sure you dont smell like a prey item, you should have no worries about being struck just by reaching in to handle them. I just tap mine on the side with my hand, away from their head, to let them know its not time to eat. Some people use a hook, or other less sensitive object.
I wouldnt worry about them ingesting any of the substrate. It will usually fall off before it hits the mouth, and if its something that is bad for them to eat, you probably shouldnt have it in their enclosure. Just my opinion.
Dustin
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1.0 Albino Green Burm
0.2 Normal Burm
1.1 Java Retic