I am not familiar enough with serial ports and the tty drivers to
  fully understand this.  However, based on what kermit reports I
  expected the TIOCMGET ioctl() to indicate all of the following bits to
  be set: TIOCM_DTR, TIOCM_RTS, TIOCM_DSR, and TIOCM_CTS.  Only the
  first two seem to be set.  I presume the lack of the others could hang
  the read() function call as it waits for the assertion of the modem
  lines.  However, the CLOCAL bit is set in c_cflag, so the modem lines
  should be ignored.  I must be confused.

I would expect those to reflect the hardware state.  The local option
in pppd affects the use of CD; termios(4) doesn't mention that
changing RTS/CTS.  Reading src/sys/dev/ic/com.c, it seems that CLOCAL
doesn't affect RTS/CTS.  So, you may also need nortscts as a pppd option.

It's odd, though, that kermit indicates that it is using rts/cts, and
shows CTS on.

Another thing to try is using /dev/dty00 instead of /dev/tty00.  This
is intended to allow locking for timeshared dial-in/dial-out of a
modem, but might help.

-- 
        Greg Troxel <gdt@ir.bbn.com>