I have come across a USB voltage logging device that I believe has the potential of logging S-meter levels over selectable time intervals. It is the Lascar EL-USB-3 Voltage USB Data Logger (http://www.amazon.com/Lascar-EL-USB-3-Voltage-Data-Logger/dp/B003DS0BS2/ref=cm_cr_pr_product_top
) and it comes with software.
The only task is to develop an acceptable hardware I/F between the FT897 meter jack and the EL-USB-3 Voltage USB Data Logger. The 897 appears to source a variable current intended to drive a 1ma analog meter directly, while the EL-USB-3 is intended to record voltage levels.