I created a dummy webpart which inherits the CoreResultsWebpart. If I add the built-in SharePoint Search SSA to the default application proxy group, this webpart renders results just like the normal Core Results WebPart.
If I however set up SP2010 against FAST instead, and add the FAST Query SSA to the default proxy group, then nothing is displayed. The oob Core Result Webpart works of course.
Either there is some quirk to get this working, or there's a bug somewhere. I've tried it on two separate farms, with the same outcome.