Hello Andras,
I did not actually solve it yet, but I think the only possible solution is to do the following:
Run your application internally with 50 frames a second. Then when the API requests the next frame from you (through ScheduledFrameCompleted(), 25 times a second), you create the “interlaced frame” yourself, by taking the last 2 full frames (which represent different points in time) and use every other line of it.
I plan to implement this in a couple of months, but would be very interested in the result if someone tried that – or found any other method.