I've found that (distressingly), the default state for a tty line appears
   to be clocal.

Are you saying the *kernel* defaults to clocal mode?  If so, I don't
believe you; you probably have the `local' flag on in /etc/ttys.

   Is there a way to set clocal in gettytab that I just didn't notice?

You can set any of the various tty flags in gettytab.  From the man
page:

     c0        num               unused     tty control flags to write
                                            messages
     c1        num               unused     tty control flags to read login
                                            name
     c2        num               unused     tty control flags to leave
                                            terminal as
     i0        num               unused     tty input flags to write messages
     i1        num               unused     tty input flags to read login name
     i2        num               unused     tty input flags to leave terminal
                                            as
     l0        num               unused     tty local flags to write messages
     l1        num               unused     tty local flags to read login name
     l2        num               unused     tty local flags to leave terminal
                                            as
     o0        num               unused     tty output flags to write messages
     o1        num               unused     tty output flags to read login
                                            name
     o2        num               unused     tty output flags to leave terminal
                                            as

Did you RTFM?  B-)