Here's an old video of them building a semihollow. It appears that the main block is maple. You would want a strong wood, since you need to put your neck joint in there, just like a solid body. Its definitely not mahogany, it's way too light colored. You can see the color difference of the mahogany kerfing below.
From what I have read, an ES335 will have a maple block and the filler piece is spruce which is kerfed to bend more easily.
If you listen closely, I think that he says they use a basswood insert for the filler section that goes between the top and the block. It's about 3:30 into the video. He also explains about doing floating blocks (top is fixed, back is not).