I don't think an older powerbook would be a good idea.
First, because you'd need some expensive controller board to connect it to the mini's VGA output, and second, if it's really old, the resolution would be 640x480 or 800x600, which isn't nice for OSX.
The best would be to try and find a VGA screen that fits it.
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So this is how PowerPC dies... with thunderous applause