Hot on the heels of the exclude from BOM feature, I just pushed a commit that implements excluding schematic symbols from the netlist when updating the board. This new feature allows adding symbols to schematics that represent items that can be used to create bill of materials for complete project assemblies for such items as mounting hardware and enclosures.
This feature required a change to the schematic file format so you wont be able to open the schematic files with older versions of Eeschema. It will be available in the next round of nightly builds.
Excluding symbols form the board update is accomplished by checking the new “Exclude from board” check box in the schematic symbol properties dialog.