Every wood type is different. I worry more about twisting, warping and splitting while drying, not the reverse. If its straight while dry it should rehydrate straight as well.

I dont have experience with moisture meters. Never needed them. Of all the tools you can blow money on in this hobby, thats about the last I would think of. They are useful for certain areas, such as research and if you are going professional and feel the need to cover every base, but they arent needed for making a bow. Expect a lot of differing opinions on any bow making subject, but thats been my experience.

So leaving that aside heres what I do and its always served me well. If you have wood that you are concerned may be too dry, take the stave down to floor tiller state and leave it in your house for a week or so. At that point the limbs will be thin enough that it can become reaccustomed to the enviorment fairly quickly.

You can also take off a good size splinter and shave it into a rough bow shape. If it breaks with a mild bend, thats a problem.

If you have an accurate digital scale you can weigh the floor tillered bow as soon as you bring it in and then continue weighing it for the next week or two. It might gain a little weight if the bow wood was in fact bone dry. This indicates the MC rising. Make sure to always store a bow stave or blank horizontal as well.

All that being said, Ive lived and made bows in some pretty hot places. My habit is to work a bow a bit and then bring it in the house and let it sit when not actually working it. I might work on 3 different bows at a time so maybe they have more time to rest in the house and be stable moisture-wise. To be honest, Ive never had a bow break from being too dry except when Ive accidentally scorched it during bending or tempering. Which has nothing to do with summer temps.

Lastly, sometimes a piece of wood with a flaw will be more tempermental to slightly lower MC. Or perhaps its a lighter SG wood. All of these things work together. I also tend to favor heavier woods, though Ive used light somewhat. All these might be factors in my experience. But like Ive said, I have personally not had that issue and I think its pretty easy to avoid by doing the above.

How did the ancients ever manage to make a bow with just a sharp rock?   

Why do you think the wood is too dry?

What kind of wood are you working with?