>         If it's any easier, I have a stock 1.0 com.c modified to include this
> feature which works with no other changes to the OS.

If by this you mean that no other files needs to be changed,
*and* you used the trick of just using a different minor
number for /dev/cuaXX devices (e.g.  MINOR(/dev/ttyXX)+128),
make sure you are handling select() right.  You can no
longer directly call ttselect() for the cua devices.  I had
to edit conf.c to use comselect() so that dev_t passed to
ttselect is that of /dev/ttyXX not /dev/cuaXX since
    cdevsw[major(device)].d_ttys[minor(device)];
does not exist for cua devices in my implementation.

Bakul