Wow - the more I think about this, the more I would not recommend anyone try this. I would think the contractor could have a difficult time pleasing a picky architect - and even if they were successful, this would become a maintenance issue for the owners.
Depending on pavement thickness I would suggest only the actual contraction joints cut at 1/4 depth. I would look at soffcuts beveled blades for decorative shallow joints.
We do a lot of this type work with our Architectural and Decorative Concrete Group. You are correct that the saw cutter must be very skilled. The non-control joint cuts need to be deep enough to keep the blade from wandering. 12"x12" is really a lot of cuts, and 16" or 24" will reduce the price and still have a similar look. In order to look like pavers, we normally do a Lithocrete finish. It simulates granite very closely at half the cost. Most often around here, the joints just fill with dirt and it is not a problem. In Cleveland, it may make sense to fill the joints, but that is added cost. The CJ should be done early, and the decorative joints could be done later, when the concrete is good and hard. The saw cutters are the key. We are fortunate to have some of the best in the business.
You are right in your assumption that you need a very skilled saw cutter to do this kind of saw cutting work. Some of the problems cutting have been that the saw cutter looks at the front of the saw on the line and forgets to look on the back of the line and consequently there is an errant cut made . . . which is hard to rectify. I attended a seminar years ago where an engineer said that concrete will actually begin its plastic shrinkage within 6 hours after concrete hits the ground. If this is correct, which I choose to assume, then getting a deep joint into concrete flatwork as soon as possible is important. We have a tool we made that mimics a saw cut and it is used like a deep jointer. We use our tool to cut at various critical points in the flatwork immediately after laying down the concrete. We first chisel the concrete 20% (or more) the thickness of the slab and then we go over it with our small jointer tool. The following day we come back, layout and saw cut both that particular tooled joint (so it matches). We have a special saw called the Slab Crab which has skate board like wheels on it and makes a nice cut, but one still needs to be very careful. There are many companies in our area that saw cut and demo for a living and it may be wise for you to sub this out to one of those experts. As far as the grouting goes I don't know how grouting the saw cuts would perform in Cleveland where you have freeze thaw cycles. Here in Santa Cruz, CA we have found that you can use an acrylic like Acryl 60 (1 part Acrylic: 3 parts water; Add color hardener of choice). Most of the time we have had luck with this process and the grout seems to stay fairly well in the saw cuts; but again I would not be so sure in Cleveland with freeze thaw. You may want to check in with local tile companies in Cleveland that grout in exteriors all the time.
