This answer actually made me find a way to do it with Thomthom's selection Toys:
First I use Select --> All on selected layers to select everything, including edges and faces inside groups and components. Then I use Select Only --> Front Default Material so that only faces without textures are selected. Groups, Components and Edges remain selected, however, so if I use the paint bucket to give my selection a texture (so that I can find the faces I am looking for with Material Maintenance), I paint these too and Material Maintenance selects them as well. Therefore I use sketchUV (with the untextured faces selected) to assign its placeholder texture to them by right-click --> Planar Map (view). Now it's easy to find these Faces with Material Maintenance, and as it is very eye-catching with it's colors and numbers, even just by looking at the model.
It sounds a little complicated when I explain it here, but it's actually quite convenient. Thanks for your help!