The plants themselves aren't going to hurt your snake, but excessively damp substrate or humidity could. How deep is the substrate? Some plants are more tolerant than others with regard to widely varying moisture levels. If your substrate is deep enough that you could have a reasonably dry surface area and not excessive humidity, but still have enough moisture at the bottom, they could possibly survive awhile even without an actual drainage layer - and still assuming they get enough light.

But you'll probably have to water the substrate periodically; if they started growing in whatever moisture the coco fiber had out of the bag, that's going to be slowly drying up unless you're spraying, watering, etc. If you have a lot of ventilation - say, a screen top (and especially if you use lamps for heat), that could conceivably work out to a basically dry surface, humidity appropriate to your snake, and enough water at the bottom for the plants. But only if you have fairly deep substrate and you pour a fair amount of water into it on a regular basis.

So yeah, the plants themselves won't hurt your snake but they also probably won't survive long unless you make a certain amount of effort to keep them alive. I'm sort of curious what they are though, and how long they last. Would you post pics at some point, and fill us in on how they do?