Fossil once attached to venti can't serve without it, so venti must arrive first.
My machine is fossil+venti, so not quite what you asked about, but the mechanism would
be similar. The server boots the kernel from 9fat. (If I wanted to pxe boot the venti,
I could push the boot server problem back to a boot server, perhaps the auth server.)
The kernel uses configuration pcfs, which contains enough in bootdir to start and run fossil,
which checks $venti (set in plan9.ini) to see if it should run that first, if local. If venti isn't
local, it sets up the network so fossil can dial it.
/sys/src/9/boot/local.c organises that. I don't think a configuration is provided
that starts just venti, but some of the pc* configurations in /sys/src/9/pc hint at ways to
make various custom systems.
(9front's /sys/src/9/boot use rc scripts, which is much easier to change, but it doesn't provide fossil at all.)