Know where the rogue is. Sometimes you can land a Cone of Cold or Frost Nova while they are in stealth. I'd recommend trying to use Cone of Cold in this way since it's got a relatively short cooldown. I would not recommend using Frost Nova in this way because is has a long cooldown, is a great kiting tool, and if you miss it's a big bummer.
You can also use a PVP trinket to dispel the Sap, and Cone of Cold/Frost Nova immediately afterwards so you don't get re-sapped.
Only thing I know that could possibly work, would be standing on a campfire, but that would be for situations where you're static with the flag, not running cross field.