availability does seem to be in my feed. The expression in the code suggst that if stock quantity is greater than 0 then <g:availability> in stock </g:availability> is generated else <g:availability>available for order</g:availability>. (line 355 of the googlefroogle.php file) (Happy to be corrected as I do not claim to have any php programming skills as such but just looking through the code this is what I have concluded.)

Make sure your admin includes the setting - Include Zero Quantity - true