I've never run a non-profit, but what's stopping the Foundation from releasing a raw dump of expenses?
Where I live, all government contracts/invoices need to be published before they come into effect. This obviously required some setup, but presumably you already have similar data as a source for the planned report, why not release it?
Because a raw dump of expenses obscures many important details, and sets people up for misunderstanding things and reporting them out of context.
I'd love to get to a point of radical and automatic transparency as you describe, but that takes a great deal of effort to do well – and the Matrix.org Foundation is still in its infancy. Gotta learn to walk before we can run!