Hi There,
I think is a strange request and this is the same for All render engines with dome and spherical , I think here you shows two different problems:
1. " black falls into the windows"
HDRI images from CGSource has black to give you just the dome light result , doesn't matter the render engine spherical or dome you will get the same result due the full black value at the of the HDRI bottom.
Solution would be use Infinity plane or ad a small value on the CGsource HDRI this doesn't matter the render engine
2."have Dome feature option"
The option dome is to mask the bottom of the HDRI, so your feedback about black windows reflections on the scene will be there with the feature right.
then you need spherical and HDRI with a ground complete black right now to react as dome.
Now I understand, by having dome feature would save time to not edit HDRI by make the bottom deep black manually on spherical HDRI to use as dome, for example multiply the HDRI with a texture top white bottom black and you have your dome, Again would be great a option Dome which does automatic.
But on your case you still going to have black windows problems, again try in Vray dome your reflection will be black. The feature is good yes Dome but doesn't resolve you problem caus ethe HDRI from CGsource is half sphere.
Best,
JO