I'm not a lawyer, but thought I'd throw two thoughts out there.

A few years ago, a Sonex builder wanted to take the plans, input them into his CNC machine, and make parts for other builders. Sonex LLC nixed that idea. They were fine with an individual builder using their plans and the builder's CNC equipment to create one set of parts for that one builder, but not for other builders. Never went to court, and I don't believe lawyers even got involved.

The second thought is that you mentioned that a builder could buy a wood kit, ship it to you, and you'd cut out the parts. Since I'm assuming you would not do that for free, this could impact the 51% aspect of the kit, since you would be performing professional build services. Not saying it would. I don't know the kit to which you refer, so I don't know how much other work is involved. But, I believe the FAA wants to see a certain amount of fabrication, not just assembly of parts.

If you're an EAA member, call the legal services department to see if they'd give you a 30 second quick determination. If you're then wanting to debate the issue with "Yeah, but..."s, you'll probably need to invest in a good aviation lawyer (who knows E/AB rules) since you'd be crafting a tap-dance around FAA wickets.

It would be interesting to see how this works out, though!