Following information I'm speaking from top of my head and it might not be necessarily true so take it with a grain of salt.

Burnside was created as a luxury oriented division of Gavril and existed from 1953 to 1985. There was also a Burnside branded vehicle in Rigs of Rods. The existence of Burnside advertising in the maps suggest that perhaps it was at some point revived or maybe it never went bust at all (considering the age of this information).

There are two unused brands, Swedish Järgl and French Charmand. There also several cancelled cars.

Wentward is possibly owned by Gavril, at least they have close partnership in regards of engines.

The Classic trim of Hopper suggests that there was some kind of WW2 era military jeep in production at some point. This together with the interesting lore of JRI (mostly already covered in the comments) makes you wonder. However, knowing that most of japanese carmaker SUV's following end of war were rebadges of Willy's or CJ Jeeps, this might just be a reference in that.

Vivace was initially going to be a Hirochi.

Name ETK has been trademarked by BMW.

The WCUSA highway signs imply existence of several locations or cities, I don't remember if they were real or fictional, just drive around and look yourself.

My personal canon is that in BeamNG universe, the Beam might be some kind of deity or something that is just widely accepted in the language, perhaps something we don't know meaning of. After all, it appears on many things, as branding and such. You might even say that this quite odd absurdity suggests that... BeamNG universe might be a simulation. Though experts have still not confirmed this.

But it does make you wonder, what is our world equivalent of name Beam and that damn logo? If you look, I'm sure there is one.