Yes, if we can find the right grommet, I can just order
#6-32x3/8" Zinc Indented Hex Washer Head Phillips Machine Screws at Fastenal, and put the grommet on that.
Dell's spec may have been to use with a different hard drive rail that they offer in other configurations. These screws really are 3/8".
The word "grommet" may be the key... I didn't think of it as a grommet but that's clearly what it is.