Unfortunately, without some crazy tweaking (like setting up a local HTTP mirror somewhere on the LAN and sharing its contents via ftp, so that a local HTTP mirror could be set up on the server), I don't see a way to solve this... :-(
Another way would be to download the vpsupd.exe (manual virus db update self-extract packages) to a machine on a LAN, sharing them via ftp and then having them executed on the server by a scheduler...