You could route fluid through an Acrylic panel, just saying
just use quick-disconnect fittings since it'd be a removable one. But price is a factor here too.
Cool idea! But I do think price and panel thickness would be an issue here....
So after going through what seems like a thousand different variations of the case and its layout I have come to the conclusion that the 4.7L layout I posted earlier is the best option to move forward with. I attempted to mount the motherboard and GPU in an opposing orientation as suggested earlier and while it certainly looked better, a few ugly issues presented themselves. The biggest issue being that I now had a large mass in (about) the center of the case that would block most of the airflow through the radiator. I tried many different variations of this layout and in each case the motherboard, PCIE slot, PCIE Riser, motherboard standoffs, and the motherboard mounting plate blocked most of the radiator surface. Flipping the radiator on its side like the original concept alleviated this issue but forced the height of the case to increase 1cm pushing it over the 5L mark and there was alot of unused dead space created.
So with those experiments behind me, I plan to move forward with the 4.7L concept with a few modifications. It looks as though I can decrease the dimensions even further to (L x H x T) 245mm x 210mm x 90mm for an overall volume of 4.63L. The fans will now be placed above the radiator and act as intakes. Fresh air will be pulled in through the top and be exhausted through vents around the bottom. This layout still does allow for the use of dual slot cards and a 1U low profile air cooler for the CPU if I want to push this thing to the max with a GTX 1080TI.
Now it is on to drawing up the first internal frame prototype for this layout!