Does anyone know what Abaqus uses for the hourglass stiffness when using a VUMAT/explicit? 

The documentation states: 

"the shear modulus is not available during the preprocessing stage of input for materials defined with user subroutine UMAT or VUMAT. Therefore, you must provide the hourglass stiffness parameters (see Methods for Suppressing Hourglass Modes) when using UMAT to define the material behavior of elements with hourglassing modes." 

But when using the input command Hourglass Stiffness to specify the stiffness there is an input file error something along the lines of "Hourglass Stiffness command is not supported with Abaqus/Explicit"