I agree wholeheartedly. It wouldn't be hard to point all owners in the right direction for learning how to maintain their own aircraft (manuals for the aircraft and engine) and to seek answers from the Manufacturer, if they need to. They don't need an exam to be able to do this. The Tech team should be on hand to offer advice, but ultimately help them source the right information.
There are too many variables out there to put all maintenance into a simple 'test of knowledge'. It doesn't help those that genuinely want to learn how.
and if someone is already competent ... then let them maintain their own, without interference. Education without enforcement. Works in the UK and as Jim says above, also in Canada.