Show or hide context menu item based on condition.

Answer 1

There is no Out of the Box option Provided by SharePoint / WSS 3.0 to dictate the Visiblity of the menu Item other than below items

1. Permission of the user on the Item (Add /Edit etc)
2. GroupID or as configured in the Feature of SharePoint
3. Status of the Items (Approved/Check In)

Two option you can try out is to do some Java Script hack using jQuery to control the MenuItem Visibility else use the SPGridView but in this case you will need to create a New Menu altogether and Adding of Menu Items using Feature is not possible.