You can try using the UV project modifier which will give you consistent islands sizes for the entire object and then when everything is finished, you can apply the modifier and it will hard-code the UV's so they can be exported.
There is a video posted by CGMasters that goes deeper into the setup. Here's a link to one of their free pages that has some UV mapping info. http://www.cgmasters.net/free-blender-3d-tutorials/page/2/
You add 6 empties and rotate them into the proper direction so they face the various axis'. You then you list them in the modifier to achieve the proper projection.
This is what it ends up looking like.
I big problem I foresee here is that once you apply the modifier, the UV's are a bit all over the place and overlapping. Whether this will be a problem or not depends on what you are doing with all this after the modeling/texturing is finished.
If you are baking global illumination, ambient occlusion, etc, You can add a second set of UV's then press Ctrl-P to re-pack them to fit the 0-1 space evenly.
Then you can add a material with the first set of UV's and textures then bake them all to the second set of UV's and now everything should be in proportion with UV's that are packed nicely for other types of baked textures.
EDIT: the .blend file is late, (there was an error) :) http://www.pasteall.org/blend/33715
If you select all the Empties and scale them; the texture scale will contract or expand as well.
An interesting note about this modifier is that you can add 10 projectors per modifer so adding two modifiers should be 20 points of reference. Maybe the nulls could be used to define a large scale terrain pattern through the space.