I've never been able to figure out a way to do it in MIDI in realtime,
but (as embarrassing as it is to admit) I was once masochistic enough to
spend about 2 hours in the score editor and event list creating a
whopping one bar of what sounded like fairly realistic guitar strumming.
TwiddlyBits/Keyfax used to sell some guitar midi sequences for $39.50
http://www.keyfax.com/keyfax/prdct.html
http://www.keyfax.com/
http://www.keyfax.com/keyfax/prdct.html
Actually, all you need is one or two good examples of a strum, then you copy it, and change the pitchs, and tweak it. We could keep some on the website for Logic_Cafe too, if people want to upload a strum or two.