Cool plank - and also I liked your cueing for the reverse swan - I always thought it was about getting the lower back down - but now I see it differently
Reverse Swan....a great exercise. Swan is an extension force exercise. Working all the spinal extensors; REVERSE swan, then, asks that the body hold the extension (while facing up...a total challenge) and then moving. An "articulated" reverse swan could be performed here, and that might invite the low back into flexion.