Depends what you mean by hobbyist. PCIe-capable FPGAs are getting cheaper and cheaper, and there are plenty of devboards in a PCIe card form factor, just bring your own bitstream and I/O. Hell, you might be able to use the open source Yosys/NextPNR toolchain to target the Lattice ECP5's PCIe SERDESes some day [1].

Nothing prevents you from emulating, I don't know, an Intel PRO/1000 or RTL8169 on one of these boards, or rolling your own board design from scratch. All the datasheets are out there.

[1] - https://github.com/ECP5-PCIe/ECP5-PCIe