According to a report in The Evening Telegraph, a deal to have a Scottish Premier League match between Dundee and Celtic played at Lincoln Financial Field in Philadelphia "are said to be 85% complete." This would be the first time an SPFL regular season match would be played outside of Scotland.
There are still significant hurdles to overcome for this to happen, but according to the article most of those have been overcome. FIFA has allowed UEFA and CONCACAF to sanction the match, and in turn UEFA has deferred to the Scottish Football Association and SPFL. With little reason to believe that the SFA or SPFL will not agree to hold an event that's sure to broaden the appeal of their teams and league while also making a ton of money, it would seem that the ball remains firmly in the court of CONCACAF and MLS.
MLS has apparently been cooperative, working with the event coordinators to plan this at a time when there are no MLS matches (as well as looking to avoid NFL and NHL games as well), however according to the article:
This is a very specific time frame, and not one I believe exists. MLS runs from March through October, while the NHL and NBA run from October through April. It'll be interesting to see if they're able to find a date with no other American sports happening at that time.
One alternative might be to do a doubleheader at the Linc with the Union, ensuring the match won't coincide with a Union game (as well as giving people who swear moving the Union to the Linc permanently is a good idea fodder for years). This wouldn't necessarily avoid conflict with an NBA or NHL game, but there are always scheduling quirks.
This would not be the first time that Celtic has played in Philadelphia, most recently facing off against Real Madrid at the Linc in 2012. Celtic also played the Philadelphia Union at PPL Park in 2010.