I found a 940 nm LED IR emitter at the maker space that is able to illuminate the Petri dish cover. At first I thought it was not working, but then another member at the maker space helped me realize the IR emitter has a narrow emittance band (18 degrees). After pointing the IR LED towards the bottom I was able to see objects inside the cover.

Fig 1)

Fig 1: 90 degree LED position is the left image and 45 degree LED position is the right image.

Interestingly the flatworms who up very bright in the IR light. 

Fig 2)

Fig 2: Hard to tell if the bright spots are the flatworms or artifacts of the water in the Petri dish.

Possible improvements

* Try a ~840 nm LED for increased brightness

* Add one or two more IR emitters

* Add sockets into the cover roof for the IR emitters

* Add a "lens shroud" around the camera hole opening