Are NSFW posts that contain characters that are owned by someone who is underaged reportable?
If so, what is the process for doing so?

I think any post (NSFW or SFW) with a character owned by an underage person is reportable. If they could otherwise file a legitimate takedown, but can't because they're under 18 and can't come here to do so, that would be a reason to remove the post regardless of it's rating. As for how to, you can Report it and explain the character owner is underage (you'll likely need to provide proof).