Can a 7 speed torpedo cable index a modern day 7 speed freewheel?
You've answered your own question.  It all comes down to cable pull, so use an accurate measuring tool like a vernier caliper, and see exactly how much cable is pulled per click.  Use a dot of white-out ("twink") to help measure accurately.

The cable itself is quite irrelevant, its the shifter on the handlebar that does the stepping, and the deraileur has a multiplier ratio.

There's a great table at http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/  but it doesn't list IGHs.

Shifter cable pull (mm) * Derailleur shift ratio  =  Cog pitch(mm)


So if your cable pull was 2.9mm per click, and the 7 speed RD mech had a 1.7:1 ratio then it should fit a 7 speed cassette/freewheel with the standard pitch of 5.0 mm