This can be done through simple eventing and a small script change.
First,
this is only if you want the Save menu to return to the map upon exiting, otherwise it will exit to the menu.
Go into Scene_Save, and find this at Line 50:
$scene = Scene_Menu.new(4)
Change it to this:
Now we'll make a Common Event that will control the button pressing. Go to your database, and in Common events, make an event called 'Save Button?'. Set the Trigger to to Parallel, and then make a switch. If this switch is off you won't be able to call the Save Menu.
In the event commands box, make a Conditional Branch, on the fourth page go to Button and select 'L' which is the 'Q' on your keyboard. Inside the 'Button' pressed branch put a script call command including this:
Now in the 'Else' branch put Exit Event Process. Now in game, turn the switch on and test it out!
That should work, I made in a little project to test it out. Enjoy!
Peace,
Mundane