It would be REALLY handy if you could craft using materials held in the trunk or any storage that is directly touching the workbench. That would make it much easier to craft as you wouldn't have to make sure that you had enough materials in your (limited) personal inventory. Internally, I'm not sure how to make it work. Perhaps if you look at the code for the 'Merge Chests' mod you could find a code example that would help you. One idea would be to (temporarily) increase the personal inventory size by the workbench storage size(s), move all of the inventory to the character--keeping track of how much of each item was transferred, then, when the player leaves the workbench, transfer everything left back to where it came from--up to each item's original amount.

In practice, I would use 'Merge Chests'/'Warehouses'/etc. to place large storage right next to the workbench--fed by belts--with 'Filter Assistant' setting how much of each material to have in the chest. That way, I could leave my personal inventory clear of any building materials, hop into the workbench, make whatever I need, then leave, easy-peasy.

Does that sound reasonable?