This thread got buried too quickly.

Anyway, here's a short and easy autohotkey script:

Download Autohotkey and install here: http://www.autohotkey.com/download/

Type the following script into notepad and save as miacalls.ahk

#IfWinActive ahk_class LeagueOfLegendsWindowClass

Numpad1::Send {ENTER}MIA TOP{ENTER}
Numpad2::Send {ENTER}MIA MID{ENTER}
Numpad3::Send {ENTER}MIA BOT{ENTER}

Return

Save it wherever and double click it before you start a game. That particular script binds the 1, 2, and 3 keys on the numpad to call mia top, mid, and bot. Just don't click it too fast and it will work fine. If you really want, you can even script a mia ping at the same time with this to make it really obvious, but that takes a little bit more work and depends on what resolution you are playing at. Just remember to turn the script off after the game.