This is possible with some custom code in your product/view. The general idea is that you have something like this:
A theme over-ride for the affected products. This need not consist of anything more than the product page phtml template files and the normal path for it, placed in app/design/frontend/default/thingy/template path. You then set your special products to this theme, the other files, e.g. for the header, footer will be picked up from the main store theme.
Set your products to use the new 'theme'.
On your custom templates you put in an extra chunk of Javascript and an extra quantity box, give it an id of 'tieredqty'. Use Prototype on DOM load event to run your script.
At the top of the script use the $('id-for-normal-qty-box').hide(); to hide the quantity box that is submitted with the form.
In your own faux quantity box add an event observer. You can make it onkeypress and perform validation to make it numbers only. You can also make it copy the value to the hidden textbox. That value can be handled by your own logic to round up to the next tier.
You will not need to worry about 'bubbling' the onChange event to the hidden form item as the price on the page does not need to be dynamically updated to account for price.