Sorry
I think it is not possible anymore to change the frame time but in iron/system/Time.hx there is some references to display frequency.
I think since the VSync uses the display frequency to limit the frame time, maybe you can do a workaround and “fake” the display frequency to 30. You may open another issue for this.
I also would like to change the fps limit, 60 fps in mobile is not a good idea 
Good look!