There seems to be very little information available about the initial tip tank design. A model aircraft forum, however, has featured photos of a model with tip tanks:
Otherwise, the only example I can find is a freighter concept with a swing-away nose:
Apparently, tip tanks aren't always the most beneficial means of extending an aircraft's range. Some Gulfstream IIs were so equipped, but rumor had it that the added drag actually resulted in a net decrease in range. And some sources report that, among fighter jets, approximately half of the fuel contained in external drop tanks is used to compensate for the added drag of the tanks themselves.
It's possible these factors combined with the likelihood of damage from ramp vehicles and equipment to kill the VC-10 tip tanks before it reached fruition.