I just formulated this in another thread about a similar topic, but it seems to be "the answer" to AdamH's response that reporting would be open to abuse: The game should, in the game log, keep track (at least internally, not necessarily visible to the players) of the timing when players make decisions.
So just as it notices that you played a Copper, it attaches a time stamp to that decision in the game log. If you wait 4 min before playing the next Copper, this will be "on record". Then games (reported or otherwise) can later be examined (by hand or automatically) by the devs. This means that accusations of stalling are *verifyable*, instead of just being accusations.