I saw your plight after the first paragraph, and my original thought was
VPN, but you said at the end that you don't like it. That's the only
real safe way I can think to do things though... either VPN, or SSH
tunneling or something along those lines.
If you can figure out the IP block that the dial ISP uses you could
limit access to those subnets, but it's still a sub-optimal solution.
You really should be doing it through some sort of authentication