A tomato trick I use for guacamole might help: dice the tomatoes & sssspinnnnnnnnnn
with a salad spinner. I also do this with Laura Colwin's Tomato Pie so it doesn't make the bottom crust soggy. But in that dish, I also line the bottom with cheese first, then add the spun tomatoes. Really seems to help.
Definitely helps keep guacamole from getting watery.