Yes, the interface MUST be up. You can plug it into the CLAN or S8400 LAN interface with a Reversing cable. I would HIGHLY recomend you put it on the network though. You have an IP enabled switch, could use IP phones/IP Softphones, and, for Administration with ASA from the customers PC(s). Also, to do firmware upgrades, you would need the CLAN and the S8400 LAN interface connected, so you can FTP firmware to the CLAN. If they won't let you put it on their network (very dumb), then at least get a small 4 port ethernet switch, for CLAN, MEDPRO, and S8300 LAN interface, then it will be at least somewhat functional for you to be able to do firmware upgrades, etc (you could also put you PC on the 4th port for FTPing, admin, etc). You also might want to let them know they paid good money for that MEDPRO, and to get some use out of it, they should consider possible future IP Softphones and IP Phones.
Mitch