It is what it is. Oddly, there are equivalent adjusters for the clutch and front brake levers usually not on a pricepoint bike like the unadjustable rear brake pedal implies it is. It's not though. Those little Clutch/FrontBrake adjuster knobs are BTW, about as simple and minimalist as I've ever seen them made. The rear brake pedal was another matter that I deemed needed a fix. Riding the bike I most likely do what everybody else does. Slide your foot outboard of the pedal so you can lower your toes to a more comfortable angle right ankle position. It is really bad with riding boots, where the angle of your ankle is pretty much determined by the stiffness of the boot. I've seen mods elsewhere where a wedge shaped spacer was "added". Too temporary -and- it's not adjustable!Lying on the building shaded cement driveway looking upward at the brake lever it was easy to envision an adjusting bolt&locknut where it's supposed to go. Took a good long look at it. Problem is there's not much material in the stop lug boss and it's angled funny - - - not perpendicular. No problem. Took the pedal off and noticed it runs dry in it's plastic female bushing over the steel drum pivot mount. That'd make it very low friction and maintenance free, except for occasionally removing muddy water residues. OK, back on topic . . . Clamped the pedal in the bench vise. Prick punched the hole and drilled it with the 11/64 hole required for the parts bin 5mm adjuster bolt that just stood out from the rest. Getting it center-of-mass, spot located, perpendicular and plumb was all "eyeball". It had to be because of so many variables. I used the old reliable "clock minutes" to visually transfer the drill angle required and drilled the hole. Here's the 2nd step, tapping the threads:
In the pic above you can see the wonky axis required so the adjuster screw ends up perpendicular to its pedal released stop surface. After installing the screw it was clear I had to create a flat surface for the locknut to bear against so it would more likely stay tight and look like BMW put it there. A big file took away the material in the way, then a smaller/finer pitch file made it flat and smoother.
Reinstalled the pedal and re-did the pedal free play adjustment required. Here it is with the (out-of-focus) pedal partially depressed showing the screw head fully releasing the brakelite's micro switch lever. The red dashes show the adjuster's pedal release resting point where the tip of the screw head rests when the brake pedal is released.
Here's what it looks like when you look up underneath the pedal at the adjuster.
Lastly, the new pedal position got adjusted, so my long ago injured ankle can find it's natural "park" position. Even with the SIDI boot on.
The whole job took about 20 minutes since everything needed was in the garage toolbox or nearby shelving. Feel so much better now but still wondering if the ~$2.80/bike BMW saved
was really worth it. 
FYI: If you decide to do this mod, good luck, but keep in mind the parts fiche shows the pedal cost at $122.92 USD.
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