I think this is an idea that could be quite interesting.
As for the plausibility of hail being strong enough to injure dinosaurs, there CAN be rather big hailstones and it is the kind of matter where I think it doesn't hurt too much (not the reader anyway) to simply presume such a hail storm could cause the respective injury. I think it would also be possible to create a scenario in which it was plausible for just Chomper to get hurt for example if everyone has taken cover but in different places (hailstorms usually come so suddenly that they wouldn't have the time to all move to the same cave) Chomper might worry about the absence of someone that against all judgment he runs of to find that someone (could be Ruby, or Ducky for example who might have taken cover under the water surface of a lake).
Large hailstorms are usually limited to relatively small areas and are therefore unlikely to cause as far reaching destruction as for example the leaf-gobblers in LBT 5 would.
We know from LBT 6 that there is kind of a "frontier" region of the Great Valley which seems to be freely accessible to sharpteeth. Maybe rather that leaving the Great Valley altogether the hail may force the leafeaters to temporarily move into that rather dangerous region. This might also give additional importance to Chomper being the one who is injured because with his excellent sense of smell (providing warning) and his ability to talk to Sharpteeth (e.g. in order to wait near the "entrance" to that part of the Great Valley to pretend that it is where he is coming from and to keep any sharpteeth from entering by telling either that there is no prey there or else some horror stories about the lone dinosaur or other leafeaters more dangerous than sharpteeth simply hunting for food want to encounter). Even some of the adult dinosaurs might see this and therefore be interested to nurse Chomper back to health.
As for that however I think it wouldn't work out too well to just have him eat some plant for him to be perfectly healthy again simply because so physical injury as such a blow to the head don't work that way (though I imagine there could be funny scenes of everyone trying to get poor injured Chomper to eat green stuff that is more likely to empty his stomach rather than providing any help.
There are more possibilities for interesting scenes with an injured Chomper. For example when delirious he might talk about stuff he wouldn't usually talk about to his friends (hunting experiences from earlier days or the like). There is also the option of the knock on the head making him forget about things, though that one is an element frequently used in many stories so I guess it would be good if one found something new about it if it was to be used.
In any case I think your scenario is one that could work out well. I'm always looking for stories that avoid the omnipresent elements of crossing over, war, attitudes very unfitting for the LBT characters and the like. I think your scenario is one in which all those elements could be avoided but still make for a good story
