@HideoK: Thanks again for your thoughts on this issue. I'll make an official post soon about the new rule. At that time, I'll contact those with a low post count to inform them of the new rule. If they don't respond by the next update, they'll be dropped. If they do respond and start posting on this site, they can stay on.
The 'tick' system is a great idea for tracking inactive members and should be easy to implement. I doubt I'll be able to fit another column into the chart that is posted to this thread, but I can definitely track it in my spreadsheet. Thanks again to fadetoone for the suggestion!