Then I suspend tip and run "slattach /dev/ttya" and get the error

     ioctl(TIOCSDTR): Inappropriate ioctl for device

Presumably, the SPARC serial driver doesn't know about that ioctl.  In
fact, the code says:

	case TIOCSDTR:
	case TIOCCDTR:
	[...]
	default:
		return (ENOTTY);

A cheesy implementation would be:

	case TIOCSDTR:
		s = splzs();
		zs_modem(cs, 1);
		splx(s);
		break;
	case TIOCCDTR:
		s = splzs();
		zs_modem(cs, 0);
		splx(s);
		break;

Note that I haven't tested this.