A very belated followup:

As of 14 December 2022, the USCG requires that, where lifeboat signal mirrors are required by US law, the signal mirror manufacturer must self-certify compliance to ISO 18813:2006 rather than the retired USCG requirement 160.020.

The ISO 18813:2006 requirements for signal mirrors are in section 4.23

To answer my original question:
"does anyone know what it requires? Specifically in the area of aiming aids?"

§4.23.1.3 requires (amongst other things) that there must be an "effective aiming means" in the center.

I see no definition of "effective aiming means", and given the apparently widespread ignorance of how "effective aiming means" work[1], I don't have a warm feeling about manufacturer self-certification. If I spot anything sold as such, I suppose I can buy it and see what I think.

Meanwhile, I see that ISO has replaced 18813:2006 by 18813:2022 - I have that on order to see if there are any changes.

In any event, as far as I can tell, the US requirements are still to ISO 18813:2006.


[1] Doug has pointed out that many commercially sold "signal mirrors" have "aiming means" that looks like legitimate aiming means, but does not function - see:
http://www.equipped.org/phony_signal_mirrors.htm