Update: broke two MoFlo lids trying to remove the poppet. Based on this image, I think I need the WaterGate lids to remove the poppet - the MoFlo doesn't appear to have a removable poppet (these lids are all options for the Purist bottles).
Looks like new bottle time (& more focus on the cap/valve design)
I don't hate the idea of a cover for the bottles. I've thought a lot about the manure, pesticides, herbicides, and everything else that ends up on a gravel road - all that ends up in your lungs, gut, eyes, sinus, and EVERY.OTHER.ORIFICE. IMHO, that craps gonna end up in your system one way or another - it's just one of joys of gravel. I typically, jet the bottle to the side first to clear the valve, then spray water into mouth without putting lips on the nozzle.