I'm not an electrical expert, but I have some basic knowledge, and I would be cautious or skeptical at best about using it. Speaker wire may be able to handle the wattage load, but would it be able to handle the amperage. You would need to know:
1) How many amps the speaker wire is rated for.
2) What is the amp load of the circuit breaker that the outlet is on?
3) How many ohms of resistance does each piece of Flexwatt provide before the electricity reaches the speaker wire?

I may be over-thinking this, but it just seems that using a different kind of wire in the system could affect the current going to that particular piece of Flexwatt, which could affect the temperature that piece of Flexwatt operates at.

Hopefully, someone with more knowledge about this type of thing will have a better answer.