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 22.214.171.124 and client-pc with interface 126.96.36.199. 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 -4 188.8.131.52:54321:192.168.3.3:22 192.168.3.3
That's all. We done:
ssh -p 54321 -i <key> 184.108.40.206 sshfs -p54321 -o IdentityDile=<key> 220.127.116.11:/path /mountpoint