Suppose we have the following network topology: server with two interfaces 192.168.3.3 and 192.168.4.1, passthrough-pc with two interfaces: 192.168.4.2 and 126.96.36.199 and client-pc with interface 188.8.131.52. Server acts as a gateway into 192.168.3.0/24 for passthrough-pc and provides ssh at 192.168.3.3:22. We want to access that ssh from client-pc. Passthrough-pc provides ssh to client.
It's quite simple. We just need ssh port forwarding feature. Do on passthrough-pc:
ssh -4 184.108.40.206:54321:192.168.3.3:22 192.168.3.3
That's all. We done:
ssh -p 54321 -i <key> 220.127.116.11 sshfs -p54321 -o IdentityDile=<key> 18.104.22.168:/path /mountpoint