Was doing it via ES File Explorer with root access and /system set to RW.Are you booting to recovery with system RW enabled?
Anyway, I just wiped my phone and used the above app mentioned (AOSP Browser installer) from the Play Store and it worked
Didn't expect the phone to actually wipe all my photos and videos though, past Motos wouldn't delete that stuff, guess that's changed. Oh well.
But holy crap... sooooo nice to have the AOSP browser. Chrome is a joke, I have no clue how anybody thinks it works fine. I've yet to find one site that didn't have at least one big issue with it using Chrome.