I've tried this, and it is a good method for making baked goods that rise well and have a good texture. The down side is that the flax seeds have a strong and distinctive flavor, which is good in things that are meant to taste granola-ish, but not so good in things with more delicate flavors. I recommend this method for baked goods which get a lot of their flavor by nuts and seeds. 1 Tbs. ground flax seeds plus 3 Tbs. This kind of tofu spoils rapidly once it's been opened, so it's good to use it all at once.
Ordinary tofu works too, the kind that needs to swim in a tub of water in the fridge.
When using non-silken tofu, soft varieties blend more smoothly than firm ones. Tofu needs to be whizzed in a blender or food processor before it is used. It also helps to crumble firmer tofu up before blending it. Blend till it's smooth and creamy, not grainy. In places with sensible measuring systems, that's 70 g tofu per 100 g of flour.