Since heroin converts to morphine in the liver, and morphine doesnt have a good oral BA, I would assume the same could be said for heroin.
The acetyl grouping on heroin(diacetylmorphine) makes it especially capable of passing the BBB quickly, which is why its most commonly injected. It also has a very high intranasal BA. Eating it is a waste, just like eating morphine is a waste.
You can also smoke heroin, but it must be pretty damn pure to get a good high off it.