Finally! That was a pretty huge effort, but the 7800GD is running with the embedded RISCV CPU doing everything. Only the basic direct ROM mapper running, but it will now be quick to get the rest working again!
I've tidied up the source code for the JagGD command line tool and have released to GitHub. Most useful for those wanting to port to other machines, but may also be handy to include functionality to talk to the JagGD from your own tools.