Well, I`ve found there's an easy and a complex way to to this. The easy way would be selecting "economy Flex" on the S7 site - that always booked me into mile-earning fare buckets. Although I have to admit that my data sample size is rather small, as I`ve only done this two times.
The complex way would be using first using ITA matrix to find the cheapest available fare with a mile-earning booking code: Just put c:s7+ /f bc=O|bc=q|bc=N|bc=S (those four are the cheapest buckets earning miles on AAdvantage) into the 'outbound routing codes' and 'return routing codes' box, select the flights you want and then go to bookwithmatrix.com and paste the matrix results page in there. With some luck, there'll be a click-through to an OTA you can purchase your flights on.
If there's no luck, well, visit a brick-and-mortar travel agent with the printout from matrix...