Pretty sure I already did
(in another thread or maybe that was someone else). But here's the details. Correct me if I'm wrong (i.e. if this page is out of date).
In a typical season, any player who is on the 40-man roster or 60-day injured list as of 11:59 p.m. ET on Aug. 31 is eligible for the postseason.
A player who doesn't meet said criteria for postseason eligibility can still be added to a team's roster in the postseason via petition to the Commissioner's Office if the player was in the organization on Aug. 31 and is replacing someone who is on the injured list and has served the minimum amount of time required for activation. (For example, a player on the 10-day injured list who has been on it for at least 10 days, or a player who has been on the 60-day injured list for at least 60 days.) Players who are acquired in September or after are ineligible.
Seems to me that ever since they changed the rules some after K-Rod you can pretty much get anyone onto the playoff roster so long as someone is injured (including, as it says above, anyone on the 60). The only guys you can't are the ones acquired on 9/1 or later.
So if the Phillies wanted to call up McGarry on the last day of the season he'd still be playoff eligible (he'd be replacing Eflin or Knebel).