Absolute positioning is not necessarily an impairment to either responsive design or accessibility - especially when, as in this case, we're focusing on the vertical position almost exclusively and doing so within a very confined area of the page. Of course, the position properties are as capable of using percentages as the dimension properties, so you can certainly use them in responsive design.
Design decisions often require balancing, and sometimes you have to be willing to sacrifice some degree of platform flexibility to achieve enhanced performance or appearance for your primary audience. I'd hate to think we always have to limit our designs just so our pages will squish down to smartphone dimensions without any zooming. On a good day, your stylesheet(s) will deal with platform differences and limitations anyway.