I've regained control of the ship!  How I did it will amaze and bewilder...
I decided to carefully read Microsoft's ARC specification document, paying
close attention to chapter 5, "System Console".  Here they say that the
escape codes for the arrow keys, etc., are supported not only by ESC+[
(which loses on the serial console), but also as 0x9b!!  Weird?  Yes.
But it works!  I was able to send 0x9b+0x42 and successfully scrolled down
on a serial console and was able to regain control of the environment vars,
et al.  This should hopefully save on major headaches for Magnum owners.
The best thing to do is either make a simple termcap entry with the modified
escape codes, or use minicom to bind the function key macros to send these
much needed escape sequences.  Next is trying to figure out how to boot without
a keyboard, which it insists on at the moment.  I'm hoping the J4 jumper has
something to do with it, as I couldn't find any settings in ARC to fix this
problem.  I'll keep everyone posted...

Occupational hazard: serial(0) and serial(1) as expressed in ARC are actually
REVERSED compared to the labelling on the back of the chassis.  Be aware...

In the next few days, I'll play with getting NetBSD up and running...

Cheers,
-Mark