Altough in the time, when the question was asked, it was really impossible,
but it is possible nowadays.
You can to both: UNIX=>TCP and UNIX=>UNIX forwarding.
For example:
ssh \
-R/var/run/mysql.sock:/var/run/mysql.sock \
-R127.0.0.1:3306:/var/run/mysql.sock \
somehost
It is possible since OpenSSH 6.7.