What sounds good over them? There's a few things, but nothing generally is a diatonic scale.

Since the chord is symmetrical, you can use Symmetrical scales over the chords. There are two basic scales. One is made up of consecutive Half and Whole steps. The other is the reverse, the Whole and Half step scale.

They are know as the Half-Whole Tone Scale and the Whole-Half Tone Scale.

You'll see that with each of them they'll land on the 1 b3 b5 bb7, Dig into them and notice that those chord steps remain the same between them but the notes between the chord tones are what changes and gives each of those scales it's unique sound.

You can also use a dim arppegio over them, that will always work since the arpeggio contains only the notes in the chord.

You can also branch out into other non-diatonic scales that have dim chords within them and use those scales over them.

Notce the most prominant sound of a dim chord of any kind is set up by the b5 interval from the root. This interval is known as the "tri-tone". This gives it it's strongest characteristic.

You should also notice that within a regular dominant 7th chord the tritone nterval lives inside the chord, between the M3 and the b7 of the chord. If you include the dominant 7th chords 5th in there to you end up with a dim triad built within a regular dominant 7th chord.

So now, you can start sounding out some of the diminished sounds played against a dominant 7th chord strting on either the M3 or the 5th or the b7.

When you do this you are borrowing a sound from the Phrygian Dominant based on the orignal root of your dominant 7th chord. The Dominant Phrygian scale is a mode starting on the 5th note of a Harmonic Minor scale. So, if your playing in Am, try a E Phrygian Dominant scale.

I did a tutorial on the Dominant Phrygian scale that covers some of the diminished sounds, Check it out. This is to show how to play with a more Middle-Eastern sound.