Logic 9 : tempo track question

[jangoux]

So, I was trying to edit a band on Logic 9 today but there was this one problem : The song has an weird structure so the strong beat starts on the beat, but then changes the strong beat to the off-beat and then it goes back to the beat. I know how to edit the tempo map but how can i just 'slide' the ruler (or whatever it is named in Logic) so after a given number of bars, the strong beat, well, advances a little bit ?

Tried using the beat mapping but it ended up screwing the tempo track.

Just to illustrate it to make it clearer, it is something like this:

Beat+snare beat+snare beat+snare Beat-then-snare beat-then-snare

I know i can do that with time warp on Nuendo, but on Logic i dont have a clue on how to do that.