@nynaevalan - I've gone ahead and created the View Manager plugin
which hopefully should do what you need. Please give any feedback on that thread.
@garcle - Kovid has accepted a patch of mine for Calibre which will display a different icon for the Connect/Share menu option on the toolbar when you are running content server (the three balls go green instead of their blue default). There is no way to get the count of connected users. So hopefully that should suit your purpose without requiring a plugin. It will be available in the next Calibre release (0.7.55).
@idolse - the diff viewer plugin idea sounds interesting but I shall leave it to you or someone else to implement it should they want to. I usually just do a quick visual scan in the ebook viewer to compare versions of a book, I haven't had the need to go quite as far as using a diff viewer but I can understand what you are looking for. In answer to your question, absolutely a plugin can launch an external app with command line arguments, tthat is exactly what the "Open With" plugin does...