For the latter questions, WebGL is rendered as an opaque bitmap in the browser (canvas element) so it amounts to an unselectable image as far as the user is concerned: It's all pixels and no text.
Ultimately it would probably feel quite unnatural compared to the native text selection of the browser. And probably would not work very well except in a small subset of browsers.
I think at some point in the future browsers and/or operating systems will ship with machine learning powered systems for recognizing text in bitmaps and for letting users copy recognized text. This is already possible today both with third party non-ML powered OCR software and with third party ML-powered software.
Is this a more likely approach than just drawing the selection highlight and emulating the whole thing in canvas?