We could do this just by diffing before/after (see bug 646622) and then we could have a croaker-like bot for missed revbumps.
Don't you think that pkgcheck warning about it is sufficient? It makes sense to leave it to maintainer's discretion to decide whether a change requires a revbump or not.
It's a fair point and I won't argue over it if nobody else agrees, but I do think there's some value in having a central way of doing this and being able to retrospectively check a list if people are having e.g. upgrade problems.