Development-wise, it's not a trivial thing to ask for. However, we have it on our radar in our long-term roadmap.
I'm changing the status to under review.
I'm giving this post a +1. This feature is highly requested by our clients. The lack of "per-ticket" permission management (other than manually adding permissions to thousands of tickets) or preserving the existing permissions from the original ticket is holding our clients back from implementing an archiving mechanism.