The auxiliary scale is not a special property of the block, but rather just a way of calculating what scale to use when initially inserting it. The two options are based on the Dimension scale and the Plot scale, and these can certainly be done with lisp. Just as a very basic example...
Code:
(defun c:insertdscale ( / )
(command "-insert" "MYBLOCK" pause (getvar "dimscale") "" 0.00)
)
(defun c:insertpscale ( / )
(vlax-invoke-method (vlax-get-property (vlax-get-property (vlax-get-acad-object) 'activedocument) 'activelayout) 'getcustomscale 'num 'den)
(command "-insert" "MYBLOCK" pause (/ num den) "" 0.00)
)