Combining infill of different density

Or, more precisely, 100% fill area with sparse infill without any loop in between. Skeinforge, at least I think, cheats here and uses dense infill from loop to loop. Slic3r, on the other hand, fills the areas independently. Its nice, as it saves time and plastic, but can it be done better? I think so.

(this is from x-carriage from Prusa-mendel Iteration II)

If the area is filled as indicated here, ie. first sparse inifill from loop to loop (red), then rest of the fully filled area, resulting part should be stronger, esp. in cases of lash on axes, where borders of independently filled areas may not touch at all. On the other hand it will most probably be uglier, and thus be reserved to "invisible layers".