I'm afraid there is no way to do that, neither override by elements, do linework nor draw detail line over them. You will only change common lines (those between/inside layers) from legend's V/G overrides panel, fill core from Type properties (only will show in coarse detail level) or even create "colors" materials to show on layers, but no change lines display. Legends components are pretty schematic elements in Revit, probably it will more useful to "fake" the legend with a detail/drafting view and make all previosuly mentioned triks to show walls the way you want to...
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