Yes it's extremely hard task for merit sources who are handing out huge merits in each Tx to different users on the forum to check for them if they are banned or not because first they have to check for quality post made by members,then also make their own post and also have IRL jobs to perform so it's time consuming task for them being merit sources to check before meriting any post.But if they have BPIP extensions installed then they would have the status of the users before hand which could help a lot.
I don't think it's too difficult when they want to post merit on posts they meet while discussing in a thread, for example the latest posts. Then something I believe is that many merit source will not send merits to currently banned users especially on their past posts, but they may have sent them before the user was banned. Lots of examples of that.
Personally, I wouldn't knowingly merit a banned member, and not because they might have made a decent post.
I'm sure you'll never get a review request from them again because they've been blocked, so maybe they'll be ignored especially since their posts have been sunk by dozens of pages (if any in megathread).