Time Capsule - Home made - OpenWrt router with usb disk acting as airport wireless device
HFS+ will get you nowhere since Linux cannot write it reliably. I don't know how to configure your OpenWRT hardware exactly but here is my solution used on both x86 Ubuntu and Raspberry Pi (I think the latter is close enough to an router - at least a smartphone)


You need some packages to make AFP and mDNS/DNS-SD work:

apt-get install avahi-daemon netatalk

Now configure your netatalk to share the volume on your Linux box. The filesystem does not make any difference, I used ext4 as it is close enough to modern HFS+ systems. (On my RPi, the only way to access files on it is to use either SSH or AFP, I uninstalled Samba)
At this point, you can hack your mac to force it to use your Linux box as Time Capsule, or you can find a real Time Capsule (or OS X Server) and replicate all its extra DNS-SD records using Avahi.