I've just released a new version of the Open With plugin that will update for Windows users the recent documents (thanks to charleski for the link). As discussed above this is not in itself a solution to cybmole's problem, but it doesn't hurt
@cybmole - I have no intention of "upgrading" the long since deprecated "Open ePub in Editor" plugin. I understand your point about not being able to have a direct button on the toolbar, but you can just assign a memorable shortcut in Open With (Alt+E works for me for Edit in Sigil) and have one less icon to find on your toolbar.
@charleski/user_none - I can't say I completely agree with the "lets not put windows specific code" in the app - I understand why you said it, but I don't agree that constraining yourself to the lowest common denominator all the time is the right thing. Like it or not, Windows is by far your largest userbase, and likely to remain so for a long time to come. One of your goals should surely be to deliver an application that should "behave properly" on Windows.
So that means that when you select a document from the Recent Items list, it should do the same call I have just added to Open With etc. You may not like having such special cases in the code, but imho it is the "right" thing to do for the Windows users.