I thought about just getting a volume dial from Amazon, but I don't know how well those would work in MAME... the resolution may not be high enough for games like Arkanoid, and while I'm told these will work as game controllers, nobody bothers to explain how.
See this thread regarding this product. In more recent years, there is a black, as opposed to the original classic aluminum, version too.
Utilizing the Griffin PowerMate is even easier and better with the latest (just tested) version of MAME (0.200). No source code changes required. I'm running Windows 10 64-bit, but these options are universal for any Windows platform. It's simply a command line switch:
mame tempest -mouseprovider dinput -mouse
The above works perfectly when setting the PowerMate application to configure the dial as follows:
While the default MAME sensitivity settings are adequate, the sensitivity of the dial is easily adjusted under the "Analog Controls" setting under MAME. To access it, press the "Tab" key while MAME is running:
After selecting Analog Controls, navigate down to "Dial Sensitivity":
Left arrow key decreases, right arrow key increases, value. Default is 100. Some may find 100 to be adequate. Setting to 150 makes it super fast and there is still plenty of room for even faster as the limit is 255.