I think John Young and others did plenty of practice aborts into the various nominated landing sites using the Shuttle sim - and also using the Gulfstream STA? - and were pretty confident about getting in if they needed to. Young pushed for a lot of other potential diversion runways to be included on the list without much success. The thought was that ditching was unlikely to be survivable but perhaps a modest overrun was acceptable.
You'd be pretty pissed off if you'd gone for broke on the flight from Santiago and the Shuttle nipped in ahead and blocked the runway! If they had used Vandenberg, I presume the Lan Chile flights would have been flown to avoid potential conflict.