wieb: I think you are right about chlorine. Bleach is rarely used in aquariums now. The exception is when 'ICH' (Ichthyophthirius sp.) lingers on.
Even the acid loving cichlids (discus) will die if a minute amount of chlorine is present in the water. It is not the acidity that kills the fish but the chlorine will affect the circulatory system causing the heart to fail.
I used lava rock ( quite expensive here) in aquariums but cleaning it is a big headache. Direct sunlight helps a lot . But short of boiling them under pressure (autoclaving) they cannot be cleaned effectively.
But as you said orchidologists will have the practical knowledge and/or experience to recommend their use with orchid medium.
To be frank I have never heard of lava rocks being used in plants before. I don't know if the high amounts of Iron (Fe++) found in them may be useful to plants
.