Just have a rough industry agreement of which features should be represented and what their symbols should be, then put them on connectors (which are pretty long, there is plenty of usable space) and on packaging and listings.
Great Idea. However, I had to write software one time that had all the possible washing symbols as input. There were 30 or 40 possible symbols. No human could possibly know all of them.
As usual though, the problem is that the industry has to come together to agree these symbols, be it formally through the USB consortium or informally in some other venue.