I know this is several years after your post, but I will post anyways (for future searchers).
First of all, according to the latest twain.h and TWAIN 2.1 spec, ICAP_AUTODISCARDBLANKPAGES was added (readded?) in TWAIN 2.0.
My Fujitsu 5120c also claims to support ICAP_AUTODISCARDBLANKPAGES, but it does not work because I am using an older TWAIN 1.7 DLL (even though setting it to TWBP_AUTO does not cause TWAIN to complain).
Thus, the only solution for my application is to analyze the sheet image to determine whether it is blank or not. Since I am working in black-and-white (1bpp), all I do is count the percentage of black pixels in the image. If it is above 1.5%, I consider the sheet not blank. This threshold value will differ based on the types of sheets you are scanning (3-5% for my sheets).