jwe wrote: "There are many possibilities for cool features like filling rectangles with a value, deleting or inserting rows or columns (more than one at a time), or inserting another matrix using a command, not just by pasting. It would be nice to have a generic interface for this so that people could easily add their own actions that are commands defined in .m files."

Deletion already has a menu item, but fill and insertion do not.

Also, a generic interface is already available if we were to use the uicontextmenu() code from figures.  The whole point of that is to be able to quickly add a right-click menu to a figure with callbacks that can be user code.