I made one years ago, though it didn't have a custom loop in it. Made my own motherboard tray, custom aluminum backplate, transplanted the power supply guts into the original Mac PSU enclosure, and even added a little board that would play the 'mac chime' when you'd power it on. Ill try and dig up some photos, if not it's collecting dust in my closet. It was a lot of work, but man is that a nicely built aluminum case to work with.
Sounds like a cool project. I used to have a Mac Pro 1,1. Upgraded it to dual quad xeons for cheap. Was a cool system until the motherboard failed and a replacement would have been more than the machine was worth. Wish I could have saved the case, but I moved shortly after and sold most of my stuff.
Eventually I will pick one up just to mod. I'm not really into Apple stuff these days, but it looks like tonymacx86 is still a thing, so a hackintosh project might be in the works.
As far as small mainstream cases that can fit a 240mm rad go, the CR280 (21.5L) is already about as small as you can get aside from the Raijintek Ophion Evo (19L).
If you want to go even smaller, there are some boutique SFF cases, but those demand substantially more money. Or you'll have to go with something off TaoBao, which has some incredible relatively affordable cases, but actually ordering one involves some hassle and risk.
I'd find some of those obscure office cases that are normally smaller, or make some case myself.