I've had some experience setting up wireless authentication as well as VPN authentication on some smaller Fortinet gateways running FortiOS 5.4 and 5.6. Most of the ecosystem within the Fortinet product line is self contained and only recently have I seen the a beginnings of some configuration options that would allow some more robust authentication options. What you are describing is a prime example of context sharing which is the essence of the pxGrid persona of ISE. Unfortunately I don't see Fortinet on the list of vendors that support integrations.
What do you envision this integration would look like? What data are you looking to consume and how would you use that data? I think Fortinet opening themselves up to 3rd party integrations would be very beneficial to their customer base, personally I'd like to see this first in their wireless implementations and the ability to leverage 3rd party portal redirects/acls and expanded RADIUS VSAs.
