I had a very similar idea to this, just this morning - glad I found this thread!
I currently have a notepad sitting next to the TV with a list of all the shows that my wife & I are currently watching, and we mark off the episodes as we watch them. After a while it becomes a bit of a mess, and requires re-drafting. Of course it's also prone to the odd human error. It makes sense that PS3MS should be able to tell me what we have/haven't watched!
My version of it goes like this...
The "seen-ness" (for lack of a better word) is instead indicated on the icon
rather than the title description. So, for example, instead of displaying the regular icon for a video file, it would display an icon that said "SEEN". Or, better yet, it would overlay a "SEEN" banner on the original graphic. Similarly, for a video that had been started but not finished, it could display "STARTED" or similar.
I do, however, like the idea of knowing when it was seen. A choice between "date last seen" and "number of days/weeks/months/years since" would be best.
Now whilst I'm a software engineer, I'm not at all familiar with PS3MS or any of the protocols involved. My immediate thoughts were that it could be done as a plug-in? And perhaps store the "seen-ness" in a file (say .PS3MSSTATUS) in each folder (simple text format, one line per video file in that folder). Absense of said file, or an entry in the file for any video in that folder, would simply assume "unseen". It would also enable you to manual "reset" the seen-ness. Of course, that was just my initial thoughts - no doubt some of you know a better way to achieve this in this case.
Would also be great to have a reporting option from the PS3MS GUI that would allow you to dump a list of all files and their "seen-ness" (hey this word is catching on!" in each of the shares. Would be cool to be able to total up the number of minutes for the seen/unseen files!
Or perhaps you're better off not knowing that!