You need a bare-minimum type of feedback.
Something that I have seen, for very low cost applications, is an on-off feedback.
Essentially, you run the push pull at maximum (50-50) duty cycle. On the secondary, you only have a comparator measuring the voltage, and when it is exceeded, it sends a signal back to fully turn off the PWM.
The results in a larger ripple voltage, but if the system tolerates this, or you are doing post-regulation, it is a simple control scheme.
The only caveat is that you must ensure that when you turn on again the PWM, the driver must be the opposite of the driver that was energized when the PWM stopped. Otherwise you risk saturating the transformer and damaging the transistors.