I can't imagine why you'd want to scallop a bass. The strings are so big I've never had a problem with the fingerboard getting in the way. It
would look cool though.
I find that using an assortment of files works well. It may take a little longer than a dremel does. When I did my guitar, I covered the frets with masking tape to protect them, then trimmed on either side so that only the fret wire was masked. If I remember correctly, I gave up on that part of the way down the neck and just decided to be careful. No problems there.
I didn't want my scallop to have it's deepest point in the middle; I wanted it right behind the fret, since that's where my finger goes anyway. To that end, I filed down to the depth I wanted with a round file, a little over a 1/4 inch in diameter. I then made the surface slope down from the preceding fret to the bottom of my first trough using an assortment of round and flat files in varying sizes. I sanded my work with medium grit sandpaper, suitable for stripping paint and leveling wood a little, and then with fine sandpaper.
