This may not fit your hiking style, so I apologize if it's not helpful. The most ultralight solution I can think of is to stop and brew a cup of tea when you want one - eliminates the need for a thermos entirely. (Don't say, "but I have to bring more fuel" - you're currently bringing the fuel to brew the tea that goes into the thermos; that just gets used at the brew-stops instead.)

Like I said, this may not fit your style. It works for me, but I'm light and slow, not light and fast. I've always got time to stop for a few minutes. I also tend not to hike in below-freezing temperatures, so if you're carrying a hot drink while hiking in bitter cold, this solution might not fit you, either.