>> discussion on adding a resistor on the RX line to pull voltage down
>> to -10 volts, when inactive.........  proposed

I think that this misses the point...  it is my understanding that the
PC actively sends a BREAK at hardware reset, ie, it pulls the TX line
to an active negative voltage for a duration longer than the transmission
of a character.  Note that this is the same voltage which is used in 
the transmission of a character.  A resistor added to hold the TX line
and prevent an active break from occuring, will also prevent any and all
characters from being transmitted, since all characters also consist of
the same active voltages as a break.

The resistor thing will work to pull a control line (not a data line)
when the cable is disconnected from the active rs232 transmitter, but
we already determined that a solution to the whole problem is disconnecting
the cable, with or without a resistor!

So the problem is not how to prevent a signal which appears to be a break
from being created by a disconnected line.... it's how to prevent a break
from being transmitted down a connected line, without killing off all the
characters (which are transmitted at the same voltage, but different
duration).

To solve that, I see three options:

1) hack the BIOS rom on the PC so a break is not sent
2) hack the hardware on the PC so a break cannot be sent
   by disconnecting that bit in the register
3) make NetBSD kernel ignore BREAK on the serial console.

Now, I like playing with hardware, but #3 is clearly the solution.