Here is the problem with deciding to start outsourcing cabinet boxes......shops are already set up to produce them, meaning we can likely produce them for the same cost as you, leaving that profit for machining parts in our pocket. Since we sell lots of cabinets, we are able to hit the ecomonies of scale to make it feasible. Lots and lots of small shops are now setup with a CNC and edgebander.
The quantity of dovetail drawers, or doors that our company would produce is very low relative to a dovetail or door outsource company. Setting up to produce dovetails or doors in house doesnt really make sense for the small shop as the ROI on the equipment to produce these will be too low. The dovetail or door company gets to spread that capitol investment over many many small companies door/drawer needs, thus the small company cant compete with the outsource company.
There is certainly a market for outsourced cabinet parts, its just not the typical cabinet shop that will be your customer. You need to find new start ups who have yet to purchase the CNC/edgebander, or another small type of cabinet company I have run across a few times. This being a company that currently sells a "factory" line of cabinets but is frustrated by the costs associated with anything that is not in their catalogue. We have a couple companies in our area that specialize on the low end market with "factory" cabinets. To hit the semi-custom market, they do exactly as you suggest and outsource everything and simply pay an employee to assemble parts.
We are a small shop, 3 of us, hitting the semi-custom market, doing just under a million in sales, at the rate of about 40 kitchens per year. We have a kitchen full of parts in a morning ready for assembly......and these parts are being machined by the same person that is assembling the last job to be cut. This is not our bottleneck. It makes no sense for us to outsource cabinet parts.
We outsource doors and drawers. At the 40 or so kitchens we do in a year, we cant justify the space, labour, machinery, or tooling costs to produce these.
Our bottleneck has always been finishing, and I would suspect this to be true for almost all small shops. We have tried outsourcing finishing, but the logistics of moving parts back and forth just didnt work very well. Growing my business would involve adding a second booth and finisher to try and keep up with the router.
The reason for my long winded reply is, as I read your original post, I suspected a bit of a sales pitch. My reply is to help you narrow your focus of customers who may actually want to purchase your components. Look to the small shop that does not have CNC, look to the small company just starting, look to the shop selling a line of "factory" cabinets and sell the system of outsourcing, look to the busy shop who may need the extra capacity.
Hope this helps, and good luck.