RecursivePrefixTreeFilter was modified in ~July 2011 to support spatial filtering of non-point indexed shapes. It seems to work when playing with the capability but it isn't tested. It really needs to be as this is a major feature.
I imagine an approach in which some randomly generated rectangles are indexed and then a randomly generated rectangle is queried. The right answer can be calculated brute-force and then compared with the filter. In order to deal with shape imprecision, the randomly generated shapes could be generated to fit a course grid (e.g. round everything to a 1 degree interval).