Of coures Z could draw his pistol while on the ground.
His “motivation factor” would be at about 9,000%.
There is an entire holster industry for this and other reasons. Every pistol has about 50 holsters made for it, to go here, there or anywhere, inside or outside the belt, on an ankle, belly, shoulder holster, plastic or leather, etc etc.
One thing they all have in common is: they make it easy to grab your blaster one-handed in a big hurry, even under extreme stress. Even on the ground.
I come from an LEO family ... father, husband, brother, nephews, cousins. There’s nothing you can teach me about holsters. Do you know what kind Zimmerman had? My father liked a shoulder holster under his shirt ... that would have been hard to access if pinned to the ground. My husband preferred his ankle holster.
There is 45 seconds of yelling for help on call 3 before a single gun shot is heard.
http://sanfordfl.gov/investigation/trayvon_martin.html
If Zimmerman is on the ground with Martin on top all that time, then it is not a quick draw situation. If Zimmerman had his weapon drawn at the beginning of the altercation, I don’t think he would be screaming for help for 45 seconds (or longer since that is just the start of the call) before firing.