get Check In Dialog projectitem VS2010

I'm building a VS2010 add-in and I want to add a function to the check-in dialog (right click on the solution node in solution explorer -> Check-In...)

I manged to add a button to the context menu that pops up when right clicking on an object but i can't figure out how to get the project item that was right clicked in the callback function (Exec).

an alternative would be to be able to use the compare command and add custom code to it, but this would probably be much harder.

Thanks ahead.

Harel M.