I got the Onhub as a BT speaker working just now. I kind of didn't take good notes, so I'll have to go back through everything later to make proper directions, but here's all the packages you'll need to install. If you're using OpenWRT official builds, you can install all the packages and kernel modules listed below and try it out that way. If you're using dadogroove's build, you'll need to follow his build directions and add them to the diffconfig, due to the added kernel modules.
kmod-bluetooth
kmod-input-uinput
kmod-sound-core
kmod-ath3k
alsa-utils
mpg-123
sbc
bluez-daemon
bluez-libs
bluez-utils
dbus
pulseaudio-daemon-avahi
pulseaudio-profiles
pulseaudio-tools
It is especially important that you install pulseaudio-daemon-avahi and not regular pulseaudio-daemon, even if you are not using avahi/zeroconf. It only says it in the package description, but this is the one compiled with support for bluez. I lost a couple hours to troubleshooting this, not proud to admit.
After that you can start to follow directions on how to link up BT to the speaker, from here:
Or other guides online for how to configure a Linux PC as a BT speaker.