I was under the impression that you where always blending certain passes at the same opacity every time. So, for instance, if you're using an image editor like photoshop; if the base color is always full opacity, put that on a layer with full opacity. If the grass layer is always at 50% opacity, put that on a layer with 50% opacity, etc. Unless I am misunderstanding you, are applying one opacity to a whole layer, or are you applying different oppacities to different points in the mesh? If so the easiest thing would be to use shaders, I imagine that you could alternatively use multi texturing, but you would end up having to use a bunch of masks and that sounds like it would get messy quickly.

Let me see if I've got this straight. Are the different heightmap fields being rendered one on top of the other, or three different fields rendering next to each other (more like tiles)? One option is to have three different grass textures with different opacities, which may seem wasteful, but in order to use the same texture and multi-texture with the three different fields, you would need three different alpha masks (if I remember correctly). The other option, again, is to use shaders, and store the various alpha values for the various textures in texture coordinates, or vertex streams.