As substances go, Heroin is not particularly hard on soft tissue. Cocaine for instance is definitely harmful to soft tissue, as it cuts off blood supply to the applied area, which over time leads to the death of the tissue. With chronic use, this often develops into severe sinus problems. Likewise, Methamphetamine does not have the same mechanism as Cocaine, though it is definitely caustic to soft tissue, as indicated by the severe burning sensation when insufflated. Heroin is not really in the same league as these two. I'm sure with repeated administration you can irritate your sinuses somewhat, but I've never heard of someone getting say, a collapsed septum as a result of insufflating Heroin.
Furthermore, you can do something that most people overlook; rinse your sinuses after you use the Heroin. This entails snorting your dosage of Heroin, waiting the necessary 15-20 minutes for absorption, then rinsing your sinuses with some warm saline. For this purpose, a Neti Pot is the simplest method in my opinion.
If you're concerned about "enzymes breaking the Heroin down" then think about how drugs survive your saliva and stomach acid while maintaining their effects. The mouth is a pretty tame environment by comparison.
If I were you, I would just make a solution for intranasal use, using saline as the vehicle. It's simpler then trying to do the sublingual route.