Running 5.0-rc5 and I can confirm that ext4 works fine. The only problem is that you can't use the ext4 partition as the cache drive. Kind of a non-starter for me.
I'm looking at automating wiper.sh. You could issue a quick "mount -o remount,ro /mnt/cache", but more likely than not you'll get a busy error. That means shutting down all processes with open files (e.g., plugins like dropbox). Probably a good idea to stop Samba also.