Connecting a modem to the console is not an issue, would be same as modem to Nexus 5K console port, As stated in the N5K hardware guide "The console port can be used to connect to a modem. If you do not connect it to a modem, connect it either before powering the switch on or after the switch has completed the boot process" This is due to setup script. Use the proper DCE connections
The better place to place a modem would be on a terminal server connected to both 6100 FI's allowing access from ethernet and dial-in
