Deliberately Crossing Safety Isolation Barrier

Let's say I have a mains referenced 400V PFC bus and I want to measure its voltage from an isolated grounded ELV or SELV.

I'm aware of many different proper isolation schemes including linearized optos, voltage to PWM converters, analog isolator IC's, isolated ADCs etc and have mocked up several of them. But I'm not liking the cost or size tradeoffs for any of them in this 'nice to have but not critical' application (particularly when considering their isolated power requirements).

But the question I'm not clear on is this: Could I just cross the isolation barrier directly with a sufficiently high impedance resistor chain (mega ohms). And if so where is this specified in the standards? Perhaps it's just the requirement of <0.5mA leakage? My understanding of safety standards is currently evolving.

Is this for a commercial product, which would be safety-agency listed?

If so, I would strongly encourage you to contact the compliance laboratory to whom you will submit the samples, and ask that same question. They may require some sort of redundancy, i.e. two or three resistors in series; and that the product still meets creepage requirements.