I don't think the Code or regs or any other IRS guidance has a clear definition of "fringe benefits." Obviously, the concept goes way beyond Section 132. Also, the IRS Pub on Fringe Benefits (15-B) has a lot of stuff that we might not consider a "fringe benefit," and that might or might not be 415 comp, e.g. stock options. Other IRS materials even list "bonuses" as a fringe benefit.
If your plan document excludes fringe benefits, I think you need to adopt a consistent policy as to what that does and does not mean and apply that.