Just a thought...

This might not help with the hierarchy situation but may help you get around your application's function limitation (which is terrible) in the future. You said the application limits you to views but can those views contain functions? You can use functions in a view. Using Jeff's suggestion: If you had a parameter table you could use a function like this:

CREATE VIEW dbo.sneakyWayToUseAFunction

AS

SELECT iTVF.*

FROM dbo.parameter

CROSS APPLY dbo.<usefulFunction>(parameter.val) AS iTVF