hmmm... CAM plants only open their stomata at night, when it is cooler and more humid. those factors minimize transpiration. that's why CAM plants are dominant in areas with scarce water. i am not sure if there is a best time of day to water CAM plants... maybe during the evening because the water will contribute to nighttime humidity without evaporating too quickly? i think watering during the evening would also suit the needs of the rupicolous laelias you are growing. just my two cents.
CAM is a water conservation adaptation in plants without much storage capacity living in environments that would favor water loss. If grown with adequate water and humidity it is basically no longer needed, but is "built in", just in case. If I'm not mistaken, many CAM plants don't have to use the process, but can.
The best time to water such plants is in the morning, so they'll have an opportunity to dry out...just like all others.
ray is right about CAM plants in general. however, certain species of orchids, such as rupicolous laelias, only get water in the evening in nature (from dew, which forms due to the higher humidity levels during the night). it's recommended to water rupicolous laelias in the afternoon or in the evening.