For one, that way of doing it would involve a lot of heavy lifting on the plugins part, and does not really describe a plugin anymore... and even then plugins have no way of reacting to filelist updates naturally as it stands (ie. they will not know when your share has been updated, outside of completed downloads, beyond monitoring the filesystem on OS level).
If you want to think of it differently, the work required to make this happen is substantial enough that at that point going the extra mile to make a fully fledged bot wouldn't be that much of an undertaking. In other words it is not impossible but it is impractical, because the plugin system couldn't really help the plugin in any significant way.
The bottom line: there are very few things that are truly impossible, when it comes to programming, but a lot more that are not possible with reasonable effort. The method you described may seem simple, but in truth it is anything but that, in comparison to limiting this hypothetical plugin to only downloaded files. Now, if API changes were made the situation might change, significantly even, but typically doing API changes for a single plugin is a bad idea (any API changes made should have more than one potential use case). Regardless, as it stands creating such a plugin at present is in fact not possible.