Based on eDio and medvegiypreved's work here: https://wiki.archlinux.org/index.php/Backlight#Backlight_PWM_modulation_frequency_.28Intel_i915_only.29 and EmlyDinesh's code to write to the memory address DON'T USE THIS AS-IS!

IntelPWMManipulation.cpp

#include <IOKit/IOService.h>

class IntelPWMManipulation : public IOService
{
	OSDeclareDefaultStructors(IntelPWMManipulation);
    
public:
	bool start(IOService *provider);
};

OSDefineMetaClassAndStructors(IntelPWMManipulation, IOService)

bool IntelPWMManipulation::start(IOService *provider)
{
	if (!IOService::start(provider))
		return false;

    IORegistryEntry *IGPU = IORegistryEntry::fromPath("IOService:/AppleACPIPlatformExpert/PCI0@0/AppleACPIPCI/IGPU");
    if (IGPU) {
        IOService *igpuService = OSDynamicCast(IOService, IGPU);
        if (igpuService && igpuService->getDeviceMemoryCount() > 0) {
            UInt32 val;
            IOMemoryDescriptor *igpuMmio = igpuService->getDeviceMemoryWithIndex(0);
            igpuMmio->readBytes(0xC8254, &val, sizeof(val));
            printf("0x%X\n", val);

            val = 0x7a107a1;
            igpuMmio->writeBytes(0xC8254, &val, sizeof(val));
        }
        IGPU->release();
    }

	return false;
}