im only familiar with jira, not jira-agile. there may be better solution for this but you can always hide things on screen depending on some conditions using javascript. i would create new plugin which contains a js script in atl.general context. that script could evaluate condition using JIRA rest APi and then just hide the comments from the dom.
Additional Thoughts:
you can specify the users or groups that meet the condition either hardcoded in script itself, or you can provide a special administration webwork that lets the administrators to set up and persist condition parameters. Then you could expose a new Rest Service which basically receives username + issue and returns boolean weather the user meets the condition to comment that issue.
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