That's not an unsolvable problem. Checking visibility is easy, computer graphics have been doing this since forever. Fonts that are too small, ditto. Contrast is well understood.

If you do want to render, you don't have to do the convoluted PDF route. That's what the user would do, the software would just use its normal drawing code. OCR is neither needed nor desirable. OCR errors would erroneously flag perfectly normal text, and it can be fooled just like a human can. You can detect whether or not there's any rendering with the bounding box of whatever text section you are looking for (something Word already has to calculate if it wants to render correctly on screen).

I'm only talking about checking for visible text. This will not solve the larger problem of malicious AI instructions.