Very risky proposal.
Forget the ftp part (it sends shivers up my spine). Put it on a normal file server on a LAN, split it, and give the users the front end. You'll have real problems hitting and Access database over a WAN or, even worse over the Internet.
An option might be to use terminal services (logmein.com, gotomypc.com, Citrix, MS Terminal Service, etc.) to allow users to access the front end remotely.