I think the best thing to do for an all-star game (or league vs league challenge) would be to do it in the preseason. There are numerous drawbacks to this, obviously, like "how do we pick the best players if they haven't played a game?" and the ever-present injury risk...but hear me out.
1. Everyone's healthy. And, with an all-star game, the rules are modified to prevent injury as much as possible anyways (no blitzing, etc).
2. Everyone's probably available and looking forward to getting out on the field. Again, a way to ease into the rigors of a season, with a few modified rules.
3. Assuming you use April as the month to hold it, ahead of the season (or seasons, depending on if it's a league challenge), it gives players the best chance to get noticed by a team in the IFL, Arena or NFL (since a couple of those leagues aren't playing and the ones that are, are always looking to sign players). April is a good month for scouting, what with the NFL Draft and all.
4. You have an entire offseason to promote the hell out of it and showcase the league's best team or best players. It becomes an EVENT, not just some token all-star game you play because it's what you do.
5. Since there is no all-star game this year, if you did it next spring, you're still giving the 2012 players a chance to be recognized in some way. Or 2012's best team in whatever scenario.
6. By the second Saturday in August, we're having fantasy football drafts, coaching at a high school or college, or calling in training camp reports from a partially-charged iPhone on a 96-degree day. In April, you're up against the NFL Draft in football.
Logistics and all that stuff--I'm not the person to ask. Call it a crazy idea after a long day at my jobs. Just my thoughts, for whatever it's worth.