Subframe keyframes didn’t work out for me, probably because raytraced motion blur only takes full frame keys into account? However, I don’t know for sure.
I see, thanks for that bit of info.
I discovered another limitation with subframe keyframes: the ‘Make Cyclic’ modifier forces the keyframes onto the frame grid. So anything in between frames doesn’t get repeated correctly.
It doesn’t matter though because your two-plane method works really well.
Now I’ll see about tweaking the materials to see if I can reduce the fireflies.