I may not understand you problem correctly, but cannot you misuse the SFTP server session option for your goal.
If I'm wrong, can you give me more specific example, how do you manage to login with SCP?
Would it be possible top honour the shell setting that is used for scp also for login into the tunnel host?
I am usually logging into two hosts before I can get to my machine, which works with scp but not with sftp.
When I select a tunnel host, I can do only one hop so I can log into the 2nd machine but not into the one that is actually the target.
I assume that the tunnel protocol uses a ssh login to forward a port to the target machine.