I don't expect the Phillies to answer the question because they have no reason to answer it yet. But they do have the ability to add J.A. Happ to their post-season roster if they want to add him, and it would not require any subterfuge to do it.
Back in the day, the only way you could add somebody who wasn't on your 25-man roster on August 31st was as an injury replacement. It led to all manner of phantom injury claims. Back in 1980, it took the Phillies the better part of two days to convince baseball commissioner Bowie Kuhn and National League president Chub Feeney that Nino Espinosa was really hurt, necessitating the addition of Marty Bystrom (5-0 in September as a call-up) to the playoff roster.
Today, the process is cleaner. Here is my understanding. On August 31st, your 25-man roster is set. At that point, your playoff-eligible players are anybody on the 25-man or anybody on the disabled list. However, those disabled list guys act almost like roster exemptions that can be cashed in during a post-season run. The Phillies have a handful of guys on the 60-day DL, including Tom Gordon. They are allowed to substitute for those disabled list guys by choosing from anybody already in the organization on August 31st. So Happ could essentially become Gordon, just for argument's sake, and then be eligible for the post-season.
Again, I don't know what's going to happen -- but that's the mechanism. A couple of people wondered if Happ might pitch a meaningless game on Sunday to end the regular season, if it is indeed meaningless. I wouldn't do it. I'd save him at this point for the post-season, either as a disaster starter, or a long rain-delay guy, or a long-relief guy, or simply as another bullpen arm.