Am i kind of missing the problem? I thought bryce effortlessly handles hundreds of millions of polygons? I mean, i've used hundreds of millions of polygons before, and apart from large file sizes (which shouldnt be a prob with todays harddisk sizes) there didnt seem to be any problems? Even with just 512mb of ram!

I think its unneccesary work to work with imagemaps and god knows other tricks to reduce polycounts if booleaning terrains works just fine... To reduce unneccesary strain just reduce the terrains resolution to the amount of detail needed. Obviously there's no point in using a 4096x4096 terrain on an object thats gunna take up 25x25 pixels...
I'd say make things high-res, save them in seperate scenes. Use families to allow for easy selecting of sets of terrains. W
hen ur deciding on final composition, import the objects one by one arrange the objects location and import the next object.
If at some point during the importing process you get the feeling that you really should get to reducing poly-count strain on your system, you can start reducing terrain resolutions as u see fit.
Objects in the distance need less detail then object in the front. objects that take up large screen areas need more detail then objects that take up little screen area.

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