shein:
I hate to say it, but I think the “powers that be” would be against it for two reasons (although I personally think it is a good idea)
1: The idea of having mass Tznius rallies and the sort is inherently not Tzanuah.
2: (and more critically) The definition of Tznius is much more vague than Lashon Hara. You will have to reach to actual Halacha (knees, elbows, neck) with none of the extras (stockings, etc.). The Chasidim etc. that are more Makpid will not sign on, nor will the Bais Yaakovs that have been preaching a more stringent dress code.
But yes, the situation is not good. Perhaps it would be better for the Klal to admit the extras are just that and strengthen the actual halachos, which I have seen that observance has gone way down over the past couple of years.
Hatzlacha!