I've never tried it, but I think it's safe (with regards to the liner). This is from the
Sigg USA website FAQ:
"Can I use my SIGG bottle for Hot Tea or Coffee?
We don't recommend you fill a lightweight aluminum bottle with a piping hot beverage, as it will render the bottle untouchable and will burn your hands if you try and pick it up. If your lightweight aluminum SIGG bottle is inside one of our insulated pouches, this will provide you with better protection, but we still don't advise it. You are better off purchasing one of our thermo bottles or our SIGG Hot and Cool Bottle for your hot beverages."
There is no mention of the liner, so it seems by omission that it has a tolerance for high temps.