How hard are they? Are they fragile?
Many of them suffer from desiccation and I wonder if they can be "sealed" in some manner. Or if the UV-sensitive crystals can be protected by some kind of coating.
It would also be interesting to see what would happen using the crystals grown in one solution as seeds in a chemically different solution. The right sequence might not be reactive, but you could layer them ...
I store my crystals in a sealed container, and they are fine after a month. People who have left them outside do not report dehydration, but the surface still turns white due to exposure to light.
Some crystals can be successfully sealed in resin to prevent dehydration. Preventing light from photodecomposing potassium ferrioxalate crystals would be very tricky. I do not know whether it is possible.
If you do this, get it fairly dry first because surface moisture will form a thin layer between the oil and the crystal making a shiny surface from different refractive indices.
Ooh, epitaxial growth! Could try that with a series of alums - Al(iii), Cr(iii) and Fe(iii) is all readily available.