I didn't get it working with SeaBIOS' SERCON, but I just moved on to SgaBIOS. It turns out it's actually better for me than SERCON, because it provides serial redirection to the point where kernel takes over, while SERCON only provides till bootloader starts.
That feature of SgaBIOS makes it possible to use full disk encryption on FreeBSD (without unencrypted /boot), because the password prompt just before bootloader is about to start and currently it's impossible to redirect it to serial (bootloader can be redirected to serial, password prompt can not). With SgaBIOS, I can get a password prompt via serial, with SERCON it's not possible (at least I didn't find a way to do so).