Tried searching through the net but couldn't find any info since the product is already discontinued. Can anyone confirm if this particular product supports IPv6?
The question should be: "Does the 100A support a FortiOS version that supports IPv6?"
That answer is yes.
Although the 100A is EOL (as of April 2009), and you should not expect support on it passed April 2014, a FortiOS version with support for the 100A and IPv6, FortiOS v4.0 MR3 Patch 15 (the latest and arguably last V4 FortiOS available), does exist and can be found on the Fortinet FTP.
All documentation that applies to and all features that are included in this FortiOS version will apply... to this FortiOS version.
Conjecture: Due to the product being EOL, you should really move away if you are using this to protect essential infrastructure that needs a high level of security; but if you're messing around and are using at home, I would suppose it should not be a problem. Fortinet allows for a credit towards an upgrade if you turn in your old hardware, or they may just allow you to keep it (as they did mine).