Thin wool works excellent in warm/hot weather. Whether or not it is THE optimal fibre can be debated. An advantage is the antismell properties where I think it beats the competition.
@Wool and Prince: Can the shirts be handwashed with a very mild detergent or just rinsed in water and air dried if needed?
Have you ever been in a really warm place with a woolen base layer? Because I have, I was in SE asia for 6 months and needless to say the merinowoolen base layer shirt I brought got used exactly one time. It's all fun and games until it gets 46 degrees centigrade and relative humidity of 100%... And another thing, wool dries a lot slower than most other fibres, which is the reason why the myth about wool being able to insulate while wet is still perpetuated. The drying actually cools you, so a faster drying shirt will cool you more than a slower drying one...
Regardless of weight, a cotton or linen, or any blended fibre shirt would probably work better. I would probably don a coolmax shirt before even thinking of wool.
The smell is a non issue compared to the dry cleaning in my book.