I think it's a good idea to have a filtering option in the front end

Yes, this is an alternative. When requested by the frontend, it could be specified whether comments should be retrieved in the feed or not. Or they are retrieved in any case and only not displayed in the frontend. There are various approaches.

to filter it at the blockchain

It happens only on Hivemind.

Ugh. I hadn't thought of that.

I can't remember why I assumed a re-sync. No comments have been resteemed yet. So, nothing would have to be replayed... I'll have to take a closer look again... my list is getting longer and longer... :-)