I managed to finally find some articles on it. It seems to be a common debate, but most seem to agree that the majority of plastics are okay for fish tanks, but it is the additives in the plastics that may be toxic. So, aromatics and other things besides the polymers.
Is Shapeways plastic pure?
I would try to test all of this out in one of my tanks. A good indicator seems to be the texture of the plastic when drying it off after being wet for while - as in, does it become brittle, or soft.
There will be a good few tests I do, but it appears that most of the plastics are aquarium safe, and that the lack of confidence in that is based on scientific studies purporting one result or another.
But there are tons of plastic products used in fish tanks. And I have never heard of anything definitive about them being dangerous. I will definitely research that more and choose my material appropriately.