While I wouldn't do it, what you're planning isn't a HORRIBLE idea, because of another handy Roth feature, pulling contributions. I suppose worst case if you needed to pay back the money in event of a job loss or such before you get hit with the huge penalty rate that usually comes with not paying off 0% interest offers on time (they frequently result in back-charges for interest for the entire interest free period as well), you can always just pull the contribution back out and pay it off. There are not tax or penalty implications to doing so.
The only risk you would take is if you had it invested and the investment was substantially down. Depending on how much is in there you may have problems pulling enough to pay off the note, or you may be forced to sell an investment when it's temporarily down, never a great situation. If you have several years of contributions already in this Roth, you probably have more than enough to pull the 5k if you really needed it.
But again, I second the people who said, don't borrow money for a stock market investment, even if the terms appear to be favorable on the surface.