I don't really see the necessity of this as a lot of times anyone on your ignore list might not even know they are on it and then they would be wondering why they can't post in a particular thread. I think it would cause an admin more issues then it would be worth.
The problem is that over the years I've gotten a few members now who each have a good deal to contribute, but for this or that reason over the years, despise eachother. I recommend the ignore list so that they don't have to be bothered by posts from someone they hate. However, the problem comes when they start a thread on one of their own projects, and the person they despise comments. Having someone on their ignore list comment on their project thread makes them feel like this person they despise is getting in through the back door. And they become aware of it whenever someone quotes or replies to that person. Somehow that really irks them to feel that the person they've ignored is now posting comments on their project thread that they won't even be aware of right away until someone else replies or quotes it. (maybe it feels like putting a sign in your yard and finding out your arch-rival has come in the night and added wording on the bottom that is hidden to you but visible to everyone else.)
I suppose this also gets into content types... a blurring of forum threads and more blog-like threads where someone wants to start a thread about their project (feeling that they want to own/direct/control their thread about their project) vs. general discussion / question threads.
I can kind of see your point as the member may feel cheated. Might be better off having a small mod done. I don't know if something could be pulled from an event listener where the mod could check to see if the original poster is on the viewers ignore list. I would post your request as a mod request as I doubt it would be anything that would be in the core.