As far as I know, the Noppoo's NKRO implementation makes it incompatible with Macs, though I could go with a weird USB to PS/2 to USB bridge if I really wanted one.
Supposedly Keycool's 21KRO does work with Macs, so that's a possibility.
Thanks for the alert about the Matias. I think I'll cross that off the list.
Any idea where to buy these? For example, the HPE is available from qtan's eBay site, but it seems he only has blue switches atm. Amazon has some Filcos for sale, but they're also blue only (at least for the full layout). mechanicalkeyboards does have all types of Keycool in stock, so that's definitely an advantage.