There's nothing in tar itself to eject the tape, but you could try using the "u" device for the tape drive. For example, say the tape device is "/dev/rmt/c0b0t4l0", you should also find that there is a "/dev/rmt/c0b0t4l0u". The "u" if for "unload", and it ejects the tape when tar, cpio, or whatever, has finished.
If it doesn't work, then it this feature is probably not supported by the tape driver that is being used.
[sig]<p> Andy Bold<br><a href=mailto: > </a><br><a href= > </a><br>"I've probably made most of the mistakes already, so hopefully you won't have to..." Me, most days.[/sig]