Unfortunately, the only solution is to cast the date as datetime. (I cannot defend that DAI V6 doesn't recognize MS-SQL DATE type as such -- since Microsoft introduced that type in 2008 as I recall.) That being said, the display and sorting of that DATE type in DAI reports does work as expected. You just don't get a date-picker on a filter using it -- unless you cast it as DateTime. AFAIK, casting date to datetime when needed is not going to impact performance. Have you experienced otherwise? I don't know why you are conflating Dataflex with Dynamic AI. They have nothing to do with each other - beyond the fact that DAI is a great solution for embedding BI, operational reporting, and ad-hoc reporting within DF Windows and webapps.
-Bob