If the sawmill doesn't have the means to fulfill these custom made parts at small/no batch sizes, a single person startup might fill the gap, and get their business just by asking nicely at the sawmill that's just not set up to help these people, but might be the first point of contact?

I have to admit I'm also unsure what GP means. :D

That's exactly what I meant, sorry that I couldn't respond earlier. It's about small batch production for custom orders. Mill owner will happily refer clients to you if you are his regular customer for wood.

Producing "cnc-milled parts, like window trims or the like"