I've also been doing some thinking on this, triggered by some recent posts here. I came up with the following:
It's just a quick rough sketch of the concept. The idea is that you can measure the balance by checking the clearance between the small bumps on the two horizontal plates and that you can balance it by shifting the upper plate. When the bumps touch, there is too much imbalance.
The springs are there to make it easier to obtain an 'almost balanced' situation, otherwise the slightest imbalance would cause the bumps to touch.
An alternative to the touching bumps approach would be an actual balancing arrow pointing down from the upper plate which should point straight down in a balanced situation.
The horizontal shifting could be done either simply moving the speaker and upper plate by hand, or you could add a threaded axis that could be turned for easy and precise adjustment of the horizontal offset.
I have drawn rollerbearings between the two plates but if you include the threaded shifting axis they may not be necessary if the friction between the two plates is low enough. In fact that would have the advantage of the horizontal shift position being secured automatically, the only way to change it is to turn the axis and that's not gonna happen spontaneously.
The angle is fixed in this drawing but obviously you can use an adjustable mechanism instead. This idea is only about
obtaining balance with any speaker regardless of its geometry and center of gravity, and how to
check that balance.
So what do you think?