There's no way to configure a category to to adhere to that behavior but a relatively simple SQL query like the following could likely accomplish what you're looking for.
Code:
SELECT PV.VariantID, PV.ProductID, PV.[Name] AS VariantName, CAT.[Name] AS CategoryName
INTO #TEMP
FROM
Product AS P
INNER JOIN ProductVariant AS PV
ON P.ProductID = PV.ProductID
INNER JOIN ProductCategory AS PC
ON P.ProductID = PC.ProductID
INNER JOIN Category AS Cat
ON PC.CategoryID = Cat.CategoryID
WHERE Cat.[Name] = 'YourCategory'
Order By PV.VariantID Desc
UPDATE ProductVariant SET IsDefault = 0 WHERE VariantID IN (SELECT VariantID FROM #TEMP)
UPDATE ProductVariant SET IsDefault = 1 WHERE VariantID IN (SELECT TOP 1 VariantID FROM #TEMP)
Dusty
AspDotNetStorefront Staff