I believe the way things are set up, this isn't possible. The Bill of Materials function will not allow either the Master or a Component item to use a 'user-specified' costing method, which is the required method for non-stock items.

What this means is that only Stock Items can be used in BOM processing. Non-stock items cannot.

If you do allow negative quantities in your Inventory, you could always create your Labour 'items' as Stock items and do BOM-building without worry of 'pre-stocking' items. Then every so often deal with those items. Note that doing this would affect all Inventory and allowing negative stock, and you'd have to be careful about Inventory valuation getting thrown by these 'labour items'. You'd need to map out how to proceed prior to setting anything up.

Also remember that to update different GL accounts for items, the GL activity is defined on the Control Account Set. This means you'd need separate control accounts for those items.