Can transformerless power supplies be UL/CSA certified and is there any specific standard they must meet?
Actually, isolation is not a must for UL certification. So yes, a transformerless regulator can be certified. You can find a lot of non-isolated UL-listed products on the market.

Probably the reviewer has no knowledge about capacitive power supplies but anyways it seems that s/he treats C2 as a "Safety-Critical Component" due to its position (i.e. mains side - after the bridge rectifier). S/he may also suggest you to use even an X/Y cap there.

In one (or some) of the EN-.... standards (I cannot really remember, sorry) safety-critical components should be defined as:


Input connectors
Fuses &amp; fuse holders
Filtering components (capacitors, chokes etc)
Varistors


etc.

From my point of view, the safety-critical components are R1, R2 and C1 (and a varistor, if any) in your circuit even if it has a zener at its output.

I think it's hard to find a standard document (because nearly all of them are protected against public access) but you can persuade the reviewer about the voltage rating about C2 by showing some measurements, test results, technical documents etc.