Hi,
I do indeed see what you mean. I your particular case, I do see the use for that capability.
The issue is that the related products listing is a listing of products and thus has been made to load products, like other products listings (if fact, it uses the same code as other products listings). So changing it to allow it to load variants as well would be in the realms of possibilities, but would require a lot of modifications and increase a lot the processing on the listings and thus reduce the speed of the processing for the products listings. So I don't see it coming, at least in the short term, especially since no one else requested such thing to be possible so far.
An alternative would be to add the HTML for the related products of each product in the description of the product and change the id of the product in the URL to the id of the variant and that way, when you click on the link to the product page of the related product, you would actually see the variant preselected on the page.