The 300 f2.8 absolutely blows both the 1.2 50 and 85 L's away when it comes to bokeh orientated portrait imagery.
But, the distance required for 300mm kills a large amount of portrait situations.
True, nothing comes without effort.
But however difficult manipulating the situation is, if you want the results that 50 and 85 lenses are incapable of delivering, whatever their speed, then do what it takes. If super bokeh and shallow dof are the primary characteristics wanted of a session then you have to go where you can use a vastly superior lens.
"...super bokeh and shallow DoF..."
Let's consider the latter. If you frame the subject the same, e.g. a full-body portrait at 2 m with the 50/1.2 or 12 m with the 300/2.8, the subject magnification is the same. So, the f/1.2 aperture of the 50L will give a thinner DoF. If the subject-to-background distance is less than ~9 m, the 50L will deliver a stronger background blur.
Obviously, that's OOF blur amount, which is distinct from bokeh.
I wondered how long it would be before somebody pointed that out. It would be interesting to know what the actual true focal length and aperture is with a 12mm tube on the 300mm f2.8.
But I digress, I, personally, prefer the combination of destroyed background, slightly deeper dof, and perspective you get from the 300, rather than the far busier background slightly shallower dof, and perspective from the short 1.2's. Though I fully understand it is all personal preference and in many cases limited by practicalities, I only ever used the 300 at one wedding but used the 50 f1.2 at lots!
P.S. The OOF blur might be more with the wider aperture, but we both know the elements of the background will be vastly bigger, and hence to the eye appear more blurred, with the 300.
No comment which better, but here is one of shot I took with 300. For creamy bokeh, I like my 85L II @ 1.2
Edit: I'm removing this photo - since I haven't ask him for permission yet