You'd think so (because that's how a normal person would design it) but no. We know from SE's previous communications that all inventory data is updated and written every 15 seconds with the auto save tick regardless of whether anything about it changed. Now I don't know exactly how their database is structured, but I promise you it makes no sense.
It should be a simple enough matter to load and edit retainer inventory during crafting. But given what SE has done lately, I'd expect there to be "technical issues" with doing it that magically disappear if you pay more for your subscription.