If the brake ferrules are hidden by the battery box then this is a serious design weakness.
Not sure that I agree with that.
As the i-Miev is a conversion of an ICE design it makes sense that the brake pipes run where they do - before the battery was added to the design they would have been just below the floorpan, and its clear from looking at these cars due to how much the battery hangs down below the car (clearly visible when standing a couple of metres to the side of the car) that the battery was designed to fit the available space under the original floorpan design and just lifts up into place under it, thus obscuring the brake pipes.
In that situation would you really want to reroute all the brake pipes below the battery where they will be subject to damage from rocks/stones, groundings on tall speed bumps etc or would you rather leave them out of harms way (at least physical harm) protected by the relatively robust battery enclosure ?
Clearly the above battery location is superior for a retrofit design where the battery hangs down a lot below the floor pan and isn't an integral structural part of a flat floor like it is in something like a Tesla.
I'd also point out that in many ICE cars its normal for the brake lines to go above the rear suspension subframe making some of the joints and part of the pipe length physically inaccessible without dropping the rear subframe. Whilst not as difficult as dropping a battery out its still a time consuming job.
One should give one's brake ferrules a good greasing once a year to avoid bi-metallic electro-corrosion.
Funny, I've been doing all my own car repairs and maintenance for over 20 years, have owned many cars in the 10-20 year old range where rust is starting to become a problem, am a regular on marque specific DIY car forums for the cars I have owned and I have never once heard the advise that brake ferrules should be greased yearly, or indeed at all.
This sounds like one of those "sounds plausible" wives tales not backed in any real evidence. Greased with what exactly ? Normal automotive grease on something exposed like the outside of a brake line under the car will wash away in no time, so is pretty pointless. You can get specialist rust protection coatings that will last a long time like "lanacoat" which is a marine grade lanoline based grease which is exceptionally clingy and sticky but its hard to come by and would look pretty messy to be honest. (I've used it on antenna's that are installed in salt spray exposed areas like boats)
The real problem here is that the pipes are probably just not protected properly, if at all, if the claims from the original post are true. Not unheard of in the ICE world either - The Series 1 Xantia had enamel coated hydraulic lines that rarely if ever failed (I still have a S1 1997 Xantia - all the brake lines are fine) but the Series 2 models skimped on the coating for the hydraulic lines and used bare steel in many places - which have a much greater tendency to rust out... but not usually until they're around 10-15 years old, and only in places like the UK that salt the roads...