Gasoline expands at a rate of 0.058%/degree F, so a tankfull on a 100 degree day that was filled from a ~60 degree underground tank willl increase in volume by 2.32% . The actual pressure developed in a closed (unvented) tank depends entirely on how full it is. A quater tank of gas has lots of vapor space which can be compressed, resulting in relatively low developed pressure. A completely full tank with little or no vapor space to compress is another story, and that seemingly small 2% increase in volume won't be denied. Whether deforming the tank, expanding the hoses, or pushing past the spring tension at the terminal end, it will find somewhere to go.
When I ran the shop for a Honda outboard dealer at the turn of the century, "making oil" was a fairly common problem, but as Wrench noted, those were all carbed engines. I earned a little bonus from the pontoon mfr we repped when I discovered a serial string of boats that had unvented fill stations installed by mistake (they looked exactly like the vented ones). These boats that had 2 strokes merely dribbled a little fuel out the exhaust, the 4 strokes "made oil".
I have no experience with fuelies, but logic dictates that the spring tension on the internal check valve(s) has to be low enough for a diaphragm type pump to easily overcome. Having never had an FI body apart though, there may be a "dead stop" there I'm not aware of.
Mostly, like you, I'm hoping for an easy fix, and a fuel vent fits that bill nicely. :-)