I was also on AD EU and yes, if you were RPing in public, it was pretty much given that you were open for random RP and the onus was on you to explain otherwise if and when you were approached. It seems alien to me for people to RP in a very public place yet refuse to engage in public RP...
I'm not sure what the solution is here. I would like to hope that most of the cases of people being ignored are down to the simple problem of having missed the approach. If that's not the case, well, I would move on and try again. Perhaps try another area altogether if that particular one seems rather insular for whatever reason.
One thing I can say is that, if you see me RPing out in public and approach me, I will absolutely respond. If I happen to be afk, I'll whisper you when I get back and apologise. If I don't respond at all then that means I somehow missed it entirely so a whisper to get my ferret-like attention would be appreciated!