The whole concept of UniWB was developed in the context of DSLRs, to allow the option to expose as far to the right as possible without clipping the highlights (http://www.guillermoluijk.com/tutorial/uniwb/index_en.htm).
So yes, if the goal is to expose to the right, based on things like the histogram that the camera shows, or the “zebras” that indicate blown highlights, then as far as I know even in today’s DSLRs the histogram and the zebras are always based on the jpeg that the camera would be saving, if you had asked the camera to save a jpeg. And the blown highlight indicators will vary depending on the picture style and also on the in-camera white balance settings.
Metering for middle gray is conceptually different from using the histogram or “zebras” to determine when the highlights of an image start to clip. And personally I’ve only ever owned and used one DSLR. And maybe I’ve completely misunderstood the question. But on my DSLR (the 2006-released Canon 400D) the metering was very much affected by the white balance.
According to this forum post: http://forum.luminous-landscape.com/index.php?topic=108758.0 the “zebra” warnings on the mirrorless Sony A7ii are less sensitive to the white balance than are DSLRs.
I have a Sony A7 mirrorless camera (the original A7), and so I just did a quick experiment, setting up a scene with an approx. 4800K LED light source on one side and an incandescent light source on the other side, and photographed the Live View with a point and shoot, setting the white balance to several of the default settings (and keeping the picture style constant). Here’s the result:
I don’t have a modern DSLR to compare to the Sony A7. And I’m too lazy to dig out the Canon 400D and take similar photographs.