I'd hazard a guess at probably not a lot. The onboard telescopes and cameras are likely calibrated to look at and focus on specific sized patches (say 15km by 15km) of the earth between certain altitudes (say LEO distance + -450m to 9000m).

Turning them to look outwards might just capture something if it happened to be inside the size of patch and whatever the telescope/camera combo can focus on (which would be the LEO distance plus or minute a bit). But space is big, even at 500km above the earth, so the likelihood of finding something in your field of view such as another satellite is probably quite low. Stars and other astronomical objects might be too dim/out of focus.

[Disclaimer: not a rocket scientist]

I don't work for Planet but I know the doves have a space-looking camera that is used to help understand the satellite's orientation based on the location of stars in the image.