I guess you might use MathJax in a browser if you create a custom html file and ask Safari to load it. However, I figure that the vast majority of the people asking for equation typesetting already do have an entire Latex pipeline installed, so it’d be easier to just use that and take a screenshot.
There are some serious drawbacks to the image-based solution, though. There wouldn’t be support for inline equations, hence one couldn’t easily write something like „this is proportional to exp(-1/k_bT)“. The second big point is that images are not editable. If one makes a mistake, one would need to retype the entire formula. I once tried to set up a keynote presentation using the new Pages latex support (which pastes the formula as an image) and it was an incredible frustrating experience.
I feel like the ideal solution would be an inline as well as a display math mode, you might enter by wrapping your equation in $ respectively $$. When one leaves the editing mode, Agenda would render the formula, when reentering the code would be displayed again.
Maybe you guys come up with an easier and more intuitive solution, but, at least for me, image support wouldn’t be able to replace native math support.