You would have to add up dt for the ticks you don't use.
Here's my idea. It adds dt to mydt every tick. Then every forth tick the "code" group is enabled which moves the objects around using mydt instead of dt. At the end of the group mydt is reset to 0. https://dl.dropboxusercontent.com/u/542 ... 15fps.capx
This could get very tedious to do. You can't use behaviors or animations with sprites, you'll have to do everything with events to ensure objects are only redrawn on that one frame. On a side note as far as the engine is concerned the fps is still 60 fps but we're only redrawing at 15fps.
I suppose you could but it wouldn't save on any performance which I assume this would be for.