This is an extension of the Custom "Open with" feature, to be able to run external tools/scripts with the selected branch/changeset(s) etc as input arguments.

For inspiration, see the "Tools - External Tools..." implementation in Microsoft Visual Studio. In that dialog, the user can add custom commands to a list that is later displayed in the menu. For each command, you can specify Title, Command Line, Initial Directory and Arguments. For the last two you can use placeholders like $(ProjectDir), $(CurLine) etc. In Plastic we could have placeholders like %wkspacepath, %repo, %selectedchangeset etc...