Tumai, I don't think I would do anything to encourage them to eat per se.
Do you have good pasture for them to graze on? If you do, that's all they'll need. If not, offer them free choice hay. Beyond that, I wouldn't give them anything grain wise. I have 2 reasons for this. 1) As you said, they will be traumatized. Adding grain to their diets could definitely cause them to colic. And I certainly wouldn't want to deal with a wild horse that is colicing.
2) If they are going to new homes, then do you really want to introduce them to a new grain/feed and then have them have to switch to another brand/type feed when they get home? Once again, asking for them to colic.
When ever we get a new horse to our ranch, they go for about a week on only pasture and/or hay. Then, once they have "settled down" they are s-l-o-w-l-y introduced to grain/feed.
It certainly won't hurt them even if they were to lose a couple of pounds.