You can achieve this quite easily. First, you need to create menu item in solution explorer context menu. You can learn how to add menu items in general on MSDN.
To be more exact, you need set your menu group's parent to:
<Parent guid="guidSHLMainMenu" id="IDM_VS_CTXT_ITEMNODE"/>
Both guid and id are predefined in Visual studio, the latter means that the menu item will be displayed whenever user clicks on standard project item. However, you need it only to appear when the name of the file ends with ".bat". This can be achived in BeforeQueryStatus method - see MSDN link above. To find out selected nodes in solution explorer, use
(UIHierarchy)ideObject.Windows.Item(EnvDTE.Constants.vsWindowKindSolutionExplorer).Object
which has SelectedItems property.