It's pretty hard on a trainer to really simulate hills. You can prop the front of the bike up and gear up, but its still not like really riding up a hill. I'd say if you are stuck on the trainer all winter work on things that will help you develop more power - so that you are hopefully improving your power to weight ratio, which will in turn improve your hill climbing (and your cycling overall).

As far as the race goes, tri's are usually no drafting events, so losing the pack isn't a concern. I'd say you may want to look at it differently. Rather than trying to "catch up" on the hills, ride more evenly, keep a tempo that is good for you and you may find that overall you are faster. If you are trying too hard to keep up with othe people as you climb, you may be wearing yourself out too quickly. If you "pop" its going to be harder to get your rhythm back when the hill is over.