Would that not slow down the app?
As long as the zip file is smaller than 4GB, would it not be better to use a tar file? The map tiles are compressed anyway, and the tar format might be less resource intensive. No unpacking, just seek and read.
tar is short for Tape ARchiver - there is no index in the header where to find what, you have to sequentially read the whole archive untill you find the file you search for. wrong file format for the goals of brut.
what we need is a archive format, which does not require memory of it's size to obtain files from it. usually .zip achives this goal, but it depends on the underlying library. so if there is a problem with big zip files, the splitting i suggested would pretty well work. not too many files, and each file not too big, fast access, and best last: if you scroll through an area, you would most likely only need tiles from one .zip file, until you hit a "border" and switch to a second .zip file.
so, best solution would be a ziplib, which doesnt require as much memory as the filesize of the zip itself, second best solution would be splitting zip's up.