Field based value lists are automatically sorted in ascending order. That's an unfortunate but built in limitation for table based value lists.
If you have a fairly small list of values and only need to use one field out of the two that are possible, then there is a trick that can be used to list the values in a arbitrary order. That does not appear to be a workable option here.
But this highlights a potentially useful detail:
No one wants to see #1, especially after you have a couple of hundred in the system.
It's possible to set up a Conditional value list that only lists some of the values from your table. There may be a date or ID number that can be used in a relationship so that only values from records with that ID number or larger, that date or later or some other determining value such as a status field in the table to limit the value list to a much shorter list of values.
In other cases, you may want to ditch the value list altogether and use a different layout design for selecting a value. A portal, for example can be set up to list values and a button in the portal row can be clicked to select a value. But now you have much more control over how the values are sorted and a descending sort order is quite possible. Just like a conditional value list, you can also filter the records displayed in the portal to reduce the number of available choices to choose.
Both such a portal or a very similar list view can be popped up in a new modal window for making such selections so that you don't have to devote layout space to the list.
Thanks Phil, I had a sneaking suspicion this could not be done. Thanks for the tips on the other options..appreciated.