Romans 5:20 could have an interesting spin on it: "Moreover the law entered, that the offence might abound. But where sin abounded, grace did much more abound." Basically, this is saying that no matter what and how strong sin 'abounds,' grace is there and stronger.
For the concept of hell, that's where all the unrepentant sinners would go (by conservative Christian views). Therefore, hell would also be thriving with sin. But the verse above seems to say that no matter where sin is, grace is also there. Nothing can separate people from the grace of God, so shouldn't God's grace also be located in hell, since there'd be a lot of obvious sin there?
Now Paul also does go on to say that simply because grace is more 'abound' does not mean that people go on gleefully sinning to make grace more prevalent, and that those who are dead to sin should not go on sinning.
However, I see no reason why that doesn't mean that God's grace "stops" at the gates of hell, or isn't in hell. It's never unavailable. Sin can't stop it, no matter what. Grace is stronger, in all locations.