You have to be on the Roster Aug 31 to be on the playoff roster unless you have an injury.
Zuckerman paints a slightly different picture. Don't know if he's right or not, I thought more like you...
Mark Zuckerman @MarkZuckerman
Yes, he is. Anybody in organization on Aug. 31 is eligible. RT @GrahamArmstrong he's not eligible
A little more: http://www.natsinsider.com/2012/08/whos-eligible-for-postseason-roster.html?m=1