There was a patch for a slightly older free version, allowing it to work with the Rift, but it was reported as not working for the newest free Unity version.
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Well, check out openFrameworks then. It's opensource and there's an Oculus add-on. Needs a bit of work but it's a good start. Or just tap into the SDK directly yourself...
As far as not wanting to go Pro for Unity... you can use a month trial and then switch to your 4-month oculus/unity trial. That's almost half a year of pro.