I gave up and disassembled this 7 pin Sargent yesterday and, interestingly, the 7th pin goes through a core change sleeve.
Unfortunately the pins shot out across the carpet while I was disassembling so I can't answer this myself:
I'm wondering if it were possible to somehow tension only the core change sleeve, would it then be possible to pick the core out of the lock by manipulating only that 7th pin?
Here are some pictures of this sleeve in case any of you haven't seen one:
Here is the plug with the sleeve on it
Here is the assembled core with the sleeve in the unlocked position.
Here is the little tab that rotates out into the cylinder to prevent the core from being extracted without the control key.
It looks like Sargent requires that the control bit positions always are keyed to a higher number for the control keys than for any other keys -
http://www.sargentlock.com/file_broker.php?document_id=2108 . If this is the case, then I guess a pin segment longer than the passageway through the sleeve is always present in the sleeve.
So it looks like my idea isn't possible but I'd like to put it out here to see if I got it right or if there are some better ideas for tricking this sleeve.