If you're talking about the usual kind of Fn key, it sends no scan code so it cannot be remapped. Instead, it tells the keyboard controller chip to switch to an alternate set of scan codes for a subset of other keys. This cannot be intercepted short of rewiring your keyboard. So no luck there, I'm afraid – unless you're feeling very handy and adventurous...  ╭(๑•﹏•)╮

Keep in mind though, that there are two Esc options on the standard Extend layer and you're free to make others. I find the Caps+Q so easy now that it's like hitting a single key. I use the pinky for Caps and ring finger for Q in this combo, of course. There's also Ext+[ (as some vimmers have Esc on LBr) but I don't use that myself.