Forgive me for sticking my nose into this Modem 7 discussion, but seems to me that this is the old fashioned way of doing things. Get a uDrive from 4DSystems. It is a postage stamp sized circuit board that accepts a micro SD card (up to 2 GB). You write all the files, that you want to move, onto the micro SD card on your PC. Then just pop the card into the uDrive, and read them off to your S-100 box. All that is required is a serial port. The uDrive has five .1" spaced pin-outs -- Tx, Rx, Gnd, 5v, and reset. The software required to understand the FAT file system on the micro SD card is in the uDrive firmware. You ask for a file, and it sends it to you. Baud rates go way up (don't know what the upper limit is, but it probably exceeds your legacy serial port capabilities. *grin*). Just configure your legacy serial port for the desired speed, and send an auto-baud command to the uDrive. Z80 apps to read from, write to, and get a directory listing of the micro SD card are on my Web site (www.hanscom.us). Rude and crude, but they work.
Easy ... easy ...
P.S. I have absolutely no connection with 4DSystems -- just a contented used of their uDrive.