There are 2 problems I see with this design:

1: You are placing the Mac Pros on their side, which may lead to premature bearing failure on the main cooling fan. Apple designed the cooling fan to be as silent as possible, which means that they optimized the bearing and the fan to work in vertical orientation. Bearings designed for thrust (vertical) orientation may not work so well if placed horizontally for a long time.

2: You are fitting triangular shaped computers, wrapped into round cases, into square shaped boxes, resulting in significant loss of space density.

Considering that Apple is a huge company that owns huge data centers, combined with the fact that it would be simply stupid for a company who makes their own OS to run anything but that OS, and combined with the above mentioned problems with using Mac Pros as server "logs" (because you cant call them blades), I would assume that Apple has internally OSX servers designed in the traditional blade configuration.

They may not sell or advertise them, but they MUST have them. Given that you guys are buying a ton of hardware, and are located nearby, and would be actively promoting running Apple hardware, wouldn't it be wise to at least approach Apple and see if they would be kind enough to sell you some of those blade form factor servers they simply must have.

I may be completely wrong here, but apple did brag about how Swift is the new language thats so flexible that you can make a Mobile app in it, or a Desktop app, or even a full blown social network. If that's the case, they must have some plans for the server market? No?

Any way, in the end it's a cool design, but I would seriously consider at least stacking the Mac Pros vertically to avoid fan issues. You can actually get a tighter form factor that way as well, unless space is not the issue. And if it's not, then hell, what's wrong with just placing a bunch of Pros on an Ikea shelf in a well air-conditioned room :)