Some authors on my list were talking about selling on partials or selling projects from proposals. One or two said they wouldn't write anything unless they already had a contract for it.
Can one query an agent with a proposal if one has a good record of publishing under one's belt, or does this apply only to editors?
Let's be clear about what a "good record in publishing" means. It means sales. Over the cash register, actual books out the door sales. Not advances, not deals: sales.
And yes. You can.
You can't sell a debut novel that way (99.99% of the time) or memoir, but almost all non-fiction is sold via proposal and sample chapter/s.
I sell client's second and third books on chapters and outlines all the time.