You cannot expand overlays length through setting its len.

Tested by putting debug messages.

>overlays.len = 0
>overlays.len = overlays_standing.len
>icon = stand_icon

>src << "[overlays.len], [overlays_standing.len], 1"

>for(var/overlay in overlays_standing)
>if(overlay)
>overlays += overlay

>src << "[overlays.len], [overlays_standing.len], 2"

0, 25, 1
9, 25, 2