I just tested this and I am noticing a few problems with it.
(1) The "repost" button appears on both the original and the reposted message even when the limit has been reached, and lets you click on it but actually does nothing. I think the button should not appear, or if it does, clicking on it should produce a message to the effect that "this message has already been posted the maximum number of times."
(2) The repost button similarly appears at the bottom of the message within the topic (as opposed to the message within the individual message list) even though it, too, does nothing and also produces no notification to the effect that it's doing nothing. It just lets you click on it and think that the message has been reposted when in fact it hasn't. The repost button should either not appear at the bottom of a message within the topic, or it should do something.
(3) The hashtag idea is great, since it makes readers aware that the repost is actually a repost. However, it implies (correctly, I see by the results) that you can only repost the first message of any topic. Two issues here: (a) the repost button appears on reply messages as well, and does nothing, and does not inform users that it does nothing; and (b) reposting the original message seems to start a separate thread, with all replies to it separated off into the original thread.
Is all this behavior really what we want? I would make the following changes:
1. Allow replies to be reposted as well as original messages in the thread.
2. Don't use a hashtag at all; instead, insert text "repost" at the top of a reposted message. (A hashtag causes the split-thread problem, in addition to implying either that you can repost only the first message in a thread, or that every message in the thread has been reposted.)
3. When the repost limit has been reached, either don't display the repost button, or display it but when clicked on inform the user they've reached the limit.
4. Allow messages to be reposted within the thread rather than only within the message itself. Alternatively, don't display the repost button on messages within a thread, because it does nothing. Alternatively, display the repost button on messages within the thread but when clicked on, inform the user that they must be within the individual message to use it.
In all these cases, I think I prefer displaying the button and then just informing users, when they click on it, that it's a no-op and include the reason. I think it could be confusing for the button to sometimes appear and sometimes not. (In fact, I'm SURE it will be confusing.) I would even go further and ALWAYS display the button (in a group that uses the feature at all), on every message individually and within every message within every thread, even after the limit, and even before the waiting period (if any) is over. When the user clicks on it, tell them "reposting this message is not allowed until the waiting period of [x] days has been reached" or "you have already reposted this message [x] times" etc.
Messages are the sole opinion of the author, especially the fishy ones.