Good idea but the project was never finished. In this project you can see how to add fields to emotion-elements (only PHP and no ExtJS!). You can use this code for your own purposes, but you have to change the plugin-namespace for productive usage.
The plugin adds the check-field Use Quantity to the product-slider emotion-element. If checked all products with more than one price (1-x and y-arbitary) will get an buy-button, to put z units of the product to the cart. The quantity z can be set via a product attribute-field.
Create text-snippets to relplace the hardcoded german text in the button-buy.tpl. The button is not using the ajax-cart and this has to be fixed. A few calculations need more logic.