Error / Hang Installing Yosemite and Mavericks on Mac Pro SSD
Not a brilliantly helpful answer, but a resolution nonetheless.

I contacted OWC's online chat tech support, who were incredibly helpful - continuing the conversation for an hour and a half - and talked me through diagnosing the problem.

The tech support rep first suggested using my old boot drive, and from there, testing whether my SSD was writable via the OS. The setup of this Mac Pro was 1 boot drive, plus 3 RAIDed drives. I found that with the RAID card still in, I couldn't access the SSD, but after removing the RAID card, and resetting the PRAM again, the SSD appeared and was certainly writable.

I don't think the RAID card was part of the original problem, because I'd tried installing without it as well, but getting feedback that I could write to the SSD was very encouraging!

I then simply tried installing Mavericks from the USB drive while already booted using the old boot drive and it worked!

So - as far as a solution for someone else encountering the same problem, I'm not sure what I can suggest, other than - if you can - boot normally, and install it that way, having selected the new drive that you want to install on.