I wrote:
Standing on the swingarm does essentially causes the rear to have two
spring rates where the difference in k is proportional to the distance
from
BB to pivot.

Sorry, this is a mistake. If the hub moves x0, the BB moves x1, the shock
compresses x2, the shock has spring constant k a the rider wieghs R, then
the work done would be:

W=x1*r+k*x2^2

This is not delta k, nor is it preload. It is more like additional unsprung
wieght.

-Dave