I'm assuming here that your LAN Macs all have IPs in the range 192.168.1.x (or in the Class A or B range reserved for private use). If you're not doing this already, you probably should be.
If that is the case, you can't directly access the target machine anyway.
What you need to do is to get your router/firewall to forward requests to port 22 to a specific machine. This needn't be the target machine as you can always SSH from there to the target machine inside the LAN.
You'll need to ensure that all the LAN machines that you might want to target have static IPs. You'd then set the router to port-forward requests onto one of them (referred to below as the primary target).
To operate, you'd just do
ssh foo.dyndns.org which will land you inside your network on the primary target, from which you can SSH to your actual target as necessary.
It's *exactly* the setup I use (with dyndns) to log into home and collect mail with pine.