However it has its downsides comparing to the non-textured automap, being "noisy" with detail is one of them.
Overaly map completely lacks anything akin to this feature which make it harder to read at times.
I think that the "filled" automap used by some other games might be a good solution to this in both automap and overlay modes, and relatively easy to implement - instead of rendering texture in this mode simply fill the field with custom color.
In the overlay mode transparency could be added to the "fill" so that it does not obscure view much, but still gives the impression of what's in- and out- of bounds.
Optionally: apply sectors light levels to the color, an alternating tinting (kinda like fake contrast), to differentiate adjascent sectors.
Here's some mockups of the idea: