We don't need such an amendment. In almost every community, there are reasonable and politically neutral laws against open flames, in order to protect public safety.
Hence, without violating anyone's rights, the public safety folks can legitimately turn fire hoses on anyone who attempts to burn a flag (or anything else) in violation of these just and reasonable laws.
I still think it should come under the fighting words statute or perhaps Inciting to Riot.
An action done knowingly to provoke a violent response.
Oh yes, it's their freedom, but what about our's?
Can't we at least set THEM on fire as a political statement?