How to do a spatial search without select() using PyQGIS?

Using QGIS 1.9.0-master, I want to do a spatial search for features in a vector layer without selecting them, i.e., without using QgsVectorLayer.select(), QgsVectorLayer.selectedFeatures() et al.

More specifically:

I want to get the feature IDs inside a rectangle. That could be accomplished with something similar to QgsSpatialIndex.intersects() if I could gain access to the spatial index of the layer's dataprovider.

Unfortunately, I didn't find a way to do it. I was only able to find how to create the index using QgsVectorDataProvider::createSpatialIndex()

Does anyone know if this is possible?