Mitch: it might be a little much to ask to borrow a lens that you have already bought for a trip to costa rica. If it was to just test out for a day or something that would be fine, but it's a risk that you might damage someone else's lens on a trip. Also, new copies of lenses aren't always better. Sometimes lenses come out with different amounts of sharpness, usually negligible, but nevertheless the promise of a new lens for that amount of time, not including taking it to a trip is big to ask for.
For me, I usually am 100% sure what I'm going to use a lens for by the time I have it in my shopping cart. If you're on the fence about any equipment, it's probably best to either rent it or go on a shoot with a friend and borrow it for a few shots.
Anyway, I know you don't mean any harm, but there's probably a very very slim chance someone would eagerly let you borrow the lens on a trip.
There's also borrowlenses.com in the south bay, they're pretty good with decent prices.
