Seems a strange place to put it. the F-101B, and several Russian/Soviet fights, Mig-29, Su-27, etc, had/have IRST. They put them on the top of the fuselage, just forward of the windscreen or just aft of the radome.
That would require having pull aircraft out of service for extended periods for modification; putting in a modified fuel tank probably takes lesser time and needs lesser rewiring.
OTOH, the Super Hornet international roadmap concept does have an integrated IRST below the nose.
http://cdn-www.airliners.net/aviation-photos/middle/5/4/8/1866845.jpg
