Yep, empty PNGs are just 103 bytes in size. But you can take advantage of preloading one at the very start, and rendering it wherever an empty tile is needed, thus conserving some time (and battery) otherwise spent on decoding. In terms of database disk size, getting rid of them would yield next to nothing.

As for sharing the pre-renders, I suspect it would take a few months to do them on commodity hardware (three days for Poland, zoom levels all the way to 15 and 16-18 for select cities); also, to achieve acceptable results, the database for the whole planet is needed (for some reason I've got proper administrative borders only around one of two countries).

Also the stylesheet I ended up with, default one from OSM-bright (which played well with imposm data) is bad, too low contrast to be viewed on a phone. Font size is also next to pathetic.