This can be done by defining a Mapping Rule for your IFC Custom Property.
Do it the following way:
1. Open the File\Interoperability\IFC\IFC Translators Dialog.
2. Click the "..." button the right of the Property Mapping field.
3. Click the "Map IFC Properties for Export" button.
4. Select the "IfcWall" item in the left pane. Then select the IFC Property in the middle pane.
5. In the upper part of the right pane, click the "New Rule" button.
6. At the bottom of the right pane, click the "Add Content" button, and select the "Wall>General>Layer" item from the list of possible element parameters.
7. Accept changes. If you did everything right, when you go back to the Wall Settings Dialog the IFC Property should inherit the Layer.
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