This module aims to replicate this functionality for any number of menu's in the Drupal menu system.
This is mainly for responsive websites, to prevent the user from needing to see all the menu items. You need to add the css yourself though to hide this and display this on certain widths. For now, I'll be presuming all responsivly themed sites have a custom theme, so presuming that isn't so invalid here.
Things on my todo list in regards to this module:
* Figure out how to make spaces in the visible items of a select list.
* Write the jQuery so that when the select is set to a value besides the default the form is submitted.
* Change the selection of what menu into what block (via the delta values).
This is based on the menu_block module, as it will only take effect on a block provided by that module. I had to limit it somewhat since I couldn't easily find a commonality between various approaches to having menu's on a site at the block level.