I'm still answering questions in terms of FileMaker 12. You never know how my answers might change as I become more familiar with FileMaker 13...
Unfortunately, Values lists that list values from a field are always sorted in ascending order. Sometimes one can set up an invisible sort field where the field has varying numbers of space characters to get a different order, but this only works for small numbers of values and only when you don't need that second field for other purposes.
To get a reverse order of selectable values may require using a different interface design such as a portal that shows the list of values set up with a button in the portal row that is clicked to select a value and enter it into the correct field. Such a portal or even just a list view layout can be popped up in a small floating window for this purpose.
Hmmm, I was thinking about a portal, since you can sort that. I'll have to give that some thought. What's funny is, this is a drop down IN a portal already!