@yang: Thanks again!
I chose bad hypothetical values, it seems. I didn't realize port 100 was privileged when I wrote my OP. I'm actually somewhere up in the thousands. 
In case it's helpful to anyone else, I ended up doing this:
Host Server
LocalForward PortOnServer 127.0.0.1:PortOnClientMachine
If I just use "localhost" instead of the IPv4 address, is ssh smart enough to use the device's IPv6 address if available? (All the machines on my local LAN have IPv6 addresses).