If you find editing filter masks without seeing mask thumbnails inconvenient, consider an alternative masking method.
- In the Layers palette, select a filtered smart object.
- In the Effects palette, double-click the "Alt-Mask" pictogram.
- Now you can mask the effect by painting on the ordinary layer mask (screenshot).
The upside of this method is that you can see a mask thumbnail and can use standard commands for handling the mask. The downside - somewhat bulkier layer structure and larger size of the saved PSD file.