The Sony Z1 does not have variable frame rates (except 60i and 50i, and a sad sad 24CF mode).
As said before, shoot 60i and deinterlace each field order, and make each field (lower and upper) individual frames, then you put them back together and you have 60 frames per second.
See this tutorial...
http://www.vimeo.com/619595
* Video was shot in HDV (60i) mode, cinema mode color settings, the exposure locked with shutter speed at 1/120.
* Captured via HDVSplit
* In after effects, import the footage into the Project pane
* Right click the footage in the project pane, go to Interpret Footage > Main
* Make sure Separate Fields (Upper) is turned on and then check Preserve Edges (Best Quality Only) on
* Made a new comp by dragging the footage to the "Create A New Composition" button
* Go to Composition > Composition Settings, set the frame rate to 23.976
* Right click on the footage in the timeline, go to Time > Time Stretch... and set the Stretch Factor to 200%
From what I see, it looks like After Effects is really generating individual frames for each slowed step.
In the video there is a large master timeclock on the top and then one in the lower right, showing the exact time of the footage as it is being slowed down. (The time of the slowed footage is not exactly half that of the project time because the slowed footage file starts something like 2 seconds before the project timeline)