I figured this out by myself... but for those who might run into the same issue, I did the following:
I removed the CSS above and replaced it with this:
Since the RadGrid comboboxes happened to have a class of RadComboBoxDropDown_WebBlue, I basically got away with a fast one. :) Dirty, but it works. I also had to set a z-index on the footer's box so that remained totally visible but the box on the grids disappeared after entering the footer.