> Is it possible for the getty to block the output?  Although, would I still
> be able to open /dev/tty00 if the getty was hogging it?

I don't know the details of the implementation.  Some devices can only be 
opened by one process at a time, others by multiple processes.

> 
> I think I forgot to mention that I'm doing all this as root...

Doesn't really matter, getty runs as root until the login is completed.

> 
> Thanks,
> David.
> 
> PS - Am I right in thinking the correct entry in /etc/ttys would be one
> that mentions /dev/tty00, but no getty, line speed, etc?  (As opposed to
> having no /dev/tty00 entry?)

No, all you have to do is make sure that it is set to "off", eg:

  tty00   "/usr/libexec/getty std.9600"   unknown off secure
                                                  ^^^

Once you have sorted that out, you might be able to set your machine up to 
use uugetty, which I think can coexist with ppp/chat by using a locking 
mechanism.

Finally, you might find it more productive (I did anyway) to write some 
simple chat scripts to talk to the serial port even when trying to debug 
things; it would certainly be easier than echo/cat etc.  Better still 
would be to get something like [xy]modem/kermit which should give you a 
nice way to talk through the serial port and see the responses.

Richard.