I want to savor-ize this cake to use tomatoes instead- any ideas for how to compensate for less sugar while maintaining good texture?

Had a look at the recipe. Tomatoes have much more liquid in them, so they are going to release their juices into the batter. My suggestion would be to make an upside-down cake. I would place tomato slices on the bottom of a well greased pan, drizzle reduced balsamic over the tomatoes. Make the batter as per the suggestion below (use half the amount of sugar) and bake. Good luck.

I've had good results (no change in texture) when cutting white sugar by as much as half.
for a a useful post and comments about this very issue from thekitchn, & referral to the great Shirley Corriher, see:
http://www.thekitchn.com/how-can-i-reduce-the-sugar-in-135433