I have the WD live hub, which has a built in hard drive and it works with the temporary subtitles. I also ran ethernet to it for network streaming, but I generally put my highest quality rips on the hub and play it from there. I really wouldn't recommend powerline adapters or wireless, they both tend to be more trouble than they are worth, and even if the conditions are perfect, 1080p streaming is still a problem. If there is any interference with a noisy appliance on the powerline or neighbors interfering with wireless devices, then your throughput goes down and it gets jittery. If you don't want to run ethernet, then stick to external hard drive, or a flash drive, or get the hub because of the built in hard drive and transfer files to it over wireless.
If you can, run ethernet because it really is worth it.