When you say “indirect light”, have you measured to determine the levels of visible light and UV? Also, what type of lighting is there? Sunlight, light from overhead room lights or interior case lights? Is there heat build-up within the cases?
There is not just light to be concerned about. Glass plate negatives require a stable environment, free of pollutants including those that might off-gas from surrounding display materials.
If these glass plate negatives are unique and irreplaceable, you could exhibit the prints pulled from them instead. It is visually more appealing to see a print than a negative.
The Canadian Conservation Institute has good basic information in Note 16/2 “Care of Black-and-White Photographic Glass Plate Negatives” found at http://www.cci-icc.gc.ca/publications/notes/16-2-eng.aspx.
Hope this helps. Good luck!