Hi.
Is the LEDE flash layout the same as the one for openwrt? What are the files which need to be writable and usually go to the JFFS2 partition? Newly installed packages go there but what about log files and maybe other ones? I think an overlayfs filesystem is mounted there. Could I just change the mountpoint to some tmpfs and write would go to ram? If this works, i guess i could remove the creation of the JFFS2 partition from the build scripts without breaking anything?!
Yes.
Have a look to the "/overlay" filesystem in your device, and see by yourself.
Modified files got to the overlay filesystem (to preserve them across reboots), temp and log files go to a tmpfs (to preserve the flash memory).