We have a "Leanpub for Causes" program that achieves this. Here's an example:
You'll see at the top right that it says "Cause Gets." In this case, the author has set it so 100% of their royalties (80% of the purchase price) goes to the cause.
In order to get this to work, first, the cause needs to sign up here and be approved by Leanpub:
To be clear, an official representative of the cause (which needs to be a registered non-profit or charity) needs to fill out the application.
Otherwise, the best option would be to make it clear in your book's "teaser" (this appears right under the title, on the book's landing page) and/or your book's subtitle and the "About the Book" information, that your intention is to give 100% of your royalties to a charity. But in that case, the pricing sliders would still show "Author Earns."