I actually think a light and variable day would be ideal for practicing with the spinnaker. Everything light and easy, no drama, etc. You could always bail and take a DNF if it turns into a complete drifter. Maybe there's a time limit and the race will be abandoned anyway.
In my experience, it's pretty rare that you get to fly the chute up and down the course without some favorable (good luck with that
) windshift. More likely you'd be able to use it on one leg, and the other would just be a bit too tight on the wind. So more than likely the scenario wouldn't really pan out.
You can actually tack an asym in real light air. But with a symmetrical chute I would probably douse and reset at the mark (if possible). This approach allows the crew to assess the new wind after making the mark rounding -- to see if the chute is even possible again. You can lose a lot more time in a chute-goat-rope-mark-rounding than by delaying the set for a minute or two.