I'll give you a wild guess.
Look up whether or not they have CAM (Crassulacean Acid Metabolism) or use the C4 pathway. I have heard pings described as succulents and they look succulent and these adaptations are common in cacti and succulents. It probably depends on what kind of ping it is. All I could find with a quick search was this preview but I ain't paying for it:
http://www.jstor.org/discover/10.2307/4 ... 4779377833
Try both ways and see what works is probably the most practical thing to do. If they do have these adaptations, possibly it would be better to spray them at night because that would be when they open their stomata, to avoid water loss during the day.
I'm wondering the same question with pineapples. The pineapple is a CAM plant so once my tops start growing new leaves I think I'll try foliar ferts an hour after lights on and an hour after lights off with 2 of them to see if it makes any difference.
I'd guess it probably won't make a big difference though.