You can "move" the song in EOF by using the "create lead silence" function which simply add silence at the start of the song.
Tuxguitar or GP aren't capable of loading an MP3 (or any song format as far as i'm aware) to check the sync of the tab. GoPlayAlong is a software made for that.
On a side note i tend to start the mp3 then just launch the GP from the place where i'm sure i am since the play function start from the point selected in the tab which isn't that complicated as long as you have some clues of where you are in the first place.
Ok Thanks. I'll check out Go Play Along. If it makes my job easier it'll be worth the $19. And I could see myself using it outside making CDLC.