You setup your WIP asset with Zero Depreciation in FAM. Then you start adding your assets every period/however often. Then once you are ready to place your assets in production create your compound assets. The value of the compound asset is just the sum of the included assets and I believe you are correct, it does not update. Thus, the best way is to approach to this process is to wait until everything is added to the project, create compound assets at that point and then transfer the compound asset to the proper asset type once it is ready to be placed in service.