Watching something in flash or a plug-in what player?
Itself, this problem was solved by appletom to gnomepanel that click blocked that the system is in standby mode. But, in the end, found/dug a direct link and watching it in normal players.
If no ready-made solutions, that comes to mind is just a crutch that would look for network in the case of activity in certain areas of block b the start of the screensaver (or is there a addon for your browser that would provide the URLs of open pages look).
In Google a lot of crutches that inhibit the screensaver if the browser is running flash. But it is, IMHO, quite dirty solution.