I think CP/NET is actually pretty close to what you're describing - what the 'server' runs on is actually arbitrary (it just needs to implement the protocol and

I think CP/NET is actually pretty close to what you're describing - what the 'server' runs on is actually arbitrary (it just needs to implement the protocol and allow file access), and there is an implementation for unix computers floating around somewhere. The clients were running CP/M 2.2 but with a kind of stub BIOS/BDOS (it sounds like it fit in a 4KB ROM of what's basically a thin client) that mostly just had the network interface code. On an FPGA, just tying together a bunch of cores with 8-bit FIFOs to communicate would be both very cheap and very high performance. I have a simple setup with a Z80 running CP/M at ~80 MHz on a Digilent "Arty" Artix-7 board, and you could probably cram 8 of them on the chip if each core only had a cache backed by DRAM.