Thanks for the comments guys!
I was referring to All-In-One (AIO) liquid coolers, not full-blown liquid cooling systems with pumps and reservoirs.
That said however, I believe a pump can fit at the bottom between the rad and a moderately sized PSU, and a reservoir can be installed on the outside at the back along the PCI slots.
The case can hold a 280mm and a 240mm rad (both with fans) at the same time while leaving a bunch of PCI slots to be used for GPUs. Depending on the motherboard and the thickness of the rad+fan assembly on top, that can mean 2 to 4 GPUs with waterblocks on them.
For the length of the cards, the total clearance for the card+rad+fan is 332mm. That means 10.5" cards should fit with no problems with regularly sized fan+rad. Longer cards would need slim fans on the rad.
For reference-height GPUs, the clearance on the side (including the shown side bracket) is 40.5 mm. With a 25mm thick fan, that leaves 15.5 mm to fit the power cable in. Tight but doable with slender cables. A slim 120mm fan on that side would help accommodate thicker cables and/or taller cards.
All GPU cards that come with an AIO solution should fit with side fans without any issues.
Aftermarket AIO kits from Corsair and EVGA should also pose no problems.
Only NZXT's Kraken G10 would forfeit the side fans.
