I might have been a liiiiittle fast out of the gates, proclaiming the new Non-Inventory items to be the next cure for AIDS.
Ralf Daugbjerg from Elbek & Vejrup tested the solution a little further than I did, and it seems that Microsoft haven’t thought this functionality through all the way.
Value Entries with a value in the “Cost-Amount (Non-Invtbl.)” field are costs, that do not create any G/L accounts according to Olof Simren’s blog post: https://www.olofsimren.com/things-to-know-about-item-charges/, they only serve to increase the cost.