That should be your solution:

http://www.conitec.net/beta/akey_lastpressed.htm

You can scan for the modifier keys with key_shift, key_control etc. The only drawback is that you will "loose" a key when the user hits two keys within one frame. That shouldn't be to likely though.

If that solution is not precise enough you can assign a function to this http://www.conitec.net/beta/aon_anykey.htm event. The scancode of the key pressed will be passed as the first argument to the function. This way you're fully independent what to do with your input and can buffer it as you see may fit your needs.