Here's my current thinking:

I've found a maker of water proof cooling fans. (Sunon 40mm x 40 mm x 20 mm)

I was originally thinking that I would use them to cool a liquid cooling radiator, but now I have a different thought:

If I build a copper heat sink "tunnel" through the middle of the case, I can seal it from the inside of the case itself, providing for airflow to cool the processor (and video I assume), but not allowing for a break in the water seal.

I'm thinking 5 of them pulling air out the top would allow for this.  Once I have the first 17", I will measure the airflow that Apple puts in it, then account for the possible failure of a fan by doubling it, as they can get blocked.