Newer versions of 60D and 90D come with serial port on the front.
See the datasheet for 60D http://www.fortinet.com/sites/default/files/productdatasheets/FortiGate-60D.pdf
That was something I immediately noticed about some of the newer desktop models and I am happy to say it' s back for most (if not all) of those units. For those that do not have serial console, you can use FortiExplorer and a USB cable to get to the boot menu where you can interrupt boot.
In fact, if you have a few of the older revisions of 60D without console port deployed remotely at a location, one possible solution is just to plug the USB into a server or even a dedicated device like a Mac Mini.
Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINETâ€” High Performance Network Security