How to regain SSH access after beeing locked out by iptables?

Unfortunately, no. Not unless you have some other management interface available like a remote serial console, iDRAC, iLO, etc. Chalk it up to experience and remember to test settings before you log out next time ;)

Assuming you can still ftp to the server, and you can log in as root via ftp (unlikely and ordinarily ill-advised) into a non-chrooted (etc) environment:

Put a file in /etc/cron.d with these contents:

* * * * * root /sbin/service iptables stop