It seems that the key issue is that people want to avoid commenting on a closed issue. I think this is best achieved by hiding any edit UI first, only enabling editing on explicit request. I suggest to put a text like

"This issue is closed. If you have a problem that appears to be related, please still submit a /new report/. If you want to provide further information on this very issue, you may still /edit it/."

where the slashed parts will be hyperlinked to appropriate URLs. Not sure how to best implement that - it may be possible to create a new template (e.g. issue.closed.html), and make that template default for closed issues.