All the replies so far have really been top notch, really like yours David.
Unusual for such solid replies to an electrical question. :)
I think it comes down to the insulation not really suited for use in a conduit due to thinness and heat buildup, as well as the possible need for a 4th wire to be safe. It would probably all work out probably more or less, but it might not be easy nor really safe nor really all that cheap, and doubtful it would be to code for any future considerations.