All I know is this- if I've taken the time to prep and paint an entire project, there is no way I'm going to try something like putting baby powder in my clear to make it matte. Why risk potentially ruining all the work you've already done, when a matte additive designed to work with my clear is available, and not that expensive?
I think Immortal Concepts has the right idea. Not all of the "old tricks" work with todays new paints. Some, like using a "burn coat" on your final coat of clear, CAN work out really well if you're careful and have a some practice at it. But, I wouldn't normally recommend the technique since it can have obvious drawbacks.
They used to use crushed fish scales or oyster shells to make pearlecent paints. Any idea how many fish it takes to have enough pearl to cover a van? Me neither, and I'm guessing that my customers may have an issue with me spending a day fishing when a deadline is looming. (I, however, am all for the fishing part, at least. In fishing vs. sanding primer, fishing wins any day. Even in Wisconsin, and yes, even in January)