But don't all the items get combined into one model at the start of the game? How does it work then? If only one of the items has the transparency flag, does the whole combined one get it? If that's the case, then they probably won't allow you to use it because of this quote that Mark.N found earlier in the thread:
Alpha channel of 32 bit "_color" texture can be used as opacity for your item if alpha is allowed for this hero.