I'm still not clear on whether this would be implemented as a member notification or an actual post to the group. The former may be more desirable so that individual members can opt out of receiving them.
There are a number of group and hashtag settings that automatically lock topics. Generating a proliferation of notifications/messages due to this could be undesirable. Anything sent should be in response to the manual locking of a topic by a Moderator. For this reason a checkbox option in some kind of new Lock Topic dialog makes sense to me.
I'll let others hash out any further details of this feature.