372         error = mount("coda", venusRoot, "coda",  MS_MGC_VAL,
      373                       islinux20 ? (void *)&kernDevice : (void
  *)&mountdata);

  in worker.cc fails - could someone quickly direct me to the actual
  definition of "mount"? It isn't the actual system-call, isn't it!?

I'm pretty sure it is; note that there is separate code for BSD and
Linux, and this is in the Linux-specific version.  The BSD44 code,
just above, use of mount matches mount(2) on NetBSD 1.6.
So I'd ktrace (or I guess ktruss on Linux) to see what's up with the
system call.

Is this error with 32-bit venus and a 64-bit kernel?  Before or after
adjusting the kernel types and rebuilding the kernel?  If the widths
are off then the kernel will find some bogus values and return EINVAL
or some such.

It might be useful to set up remote gdb so you can see what's going on
in the linux kernel coda support code, too.