Very insightful question! I just came across this helpful observation by Stephen Proskauer, MD.

"The Purpose of Living in Linear Time"

"In the dimension of physical manifestation, our experiences only appear sequential. They are separated by time and space so that all the variations on the underlying themes determined by the quantum pilot wave occupy the stage by turns in a sequence that reflects the karmic learning process. This separation by sequencing in linear time allows each variation on the karmic theme to be explored fully, each in a different lifetime. The cumulative experiences of all these lifetimes hang together like pearls on the string of quantum memory. Keep in mind that the concept of karmic learning as a linear process only makes sense in the time-bound material world of ordinary reality."

I would agree that linear time is only a product of the physical plane, but couldn't karma also be seen as that master learning plan (Proskauer's "quantum pilot wave") that exists in the non-physical plane as a unity of cause and effect, two sides of the same coin, the yin and yang? It would then manifest in the physical plane of our lives. If one agrees with Proskauer that "our experiences only appear sequential" then we could allow that in a non-physical, non-linear plane our experiences exist all at once and karma would be the summation of all the lessons learned though these experiences, the "pearls on the string of quantum memory."