Heck, call it "performance art" and you'll get a gummint grant to do it. Some people just don't know how to play the angles...
What if there is snow on the ground and you are "writing" something? Free speech!!!
Since the method by which you "write" is not permanent perhaps this is a protected form of free speech.
ahem...
2 guys at some MTV event did this in their "act" (a gong show type audience rating contest) and the woman that participated sued.
Maybe you need a gub'mint grant after all. G.G. Allen did this sort of thing in bars across the country at his "concerts" (he would even chase down audience members) without a grant though so who knows (he did end up doing some jailtime for some sort of offenses and later died after a show).
What's the 9th Circuit Court Of Appeals doing today? Maybe they can determine if this is "free speech".