Bit of a wild theory, but when the change to the spare wheel well was first spotted, someone on this forum suggested that Honda had already changed the tooling ready for the Mk4, ie the MY20 Mk3s were actually built on a Mk4 floorpan. If so, that bracket could actually be for the HV battery ... which won't actually be installed on these cars. In some circles that's called "fitted for but not with". So why fit the bracket? It looks fairly chunky, maybe it's needed to add some stiffness to that bit of structure. Maybe someone with a Mk4 could have look underneath and tell us if they have that bracket, and if it's got the battery in it?