When a player hovers that unconventional pick, and doesn't say anything, almost everyone has at least one moment of "this match is going to suck." People dread being trapped in a match for at least 15 minutes with someone who is going to do everything in their power to ruin that game. But you are very, very right. Communication goes such a long way. Just taking the time to type something in chat telling your team that you know what you're wanting to play isn't meta, but that you have a plan with it, or that you have a history of success with it, can go a long way to soothe that feeling of dread. There was actually an AD Bard top in one of my matches last night. From what we could see in /all chat (he was on the enemy team) there didn't seem to be a problem with it, and we were playing with a 4 man premade, so he likely was playing with some friends as well. But even if he wasn't, it didn't seem to cause enough problems for his team to spill over into the "report this troll top. auto locked bard. etc..." that you sometimes see.
In the rare ranked matches that I play, and the normals that I play without my friends, I get asked periodically if I would play X champion as a support. Ranked is one of the only places where I do tend to hover my champion, just so my team can get an idea of what we're looking at as a team comp; I don't tend to worry about it quite as rigorously in normals. I've one-tricked Sona for a long time now, and I'll be asked things like "can you play a tank?" "can you play an aggressive support?" "NO SONA NOOOOOO SOOOOONNNNNAAAA" (that was a personal favorite) "can you play thresh instead?" And I don't mind being asked this. If they ask nicely, I'm willing to discuss and compromise in a way that will benefit the team and help our chances to win. When I'm asked to play a tank, I'll let them know that I can play Leona, but it's been a few months, and I'm rusty, but I will let them decide whether they want to take that chance. If they insist on Leona then they already know that I'm not going to be making any pro-level plays with her, and it was their choice for me to play her. If they ask for a more aggressive support, I let them know that I am a Sona one-trick, and do like to play her as an aggressive poke-heavy presence in lane, but that I can play Karma or Syndra if they want more damage. I picked up Syndra because people kept picking Sona in my matches and I was annoyed. So I decided to punish them for taking my main. When people ask for Thresh, I tell them something similar to what I told them for Leona. I can play him, but I'm nowhere near as good with him as I am with Sona, and while I will try my best to land his cc, I will miss hooks, and probably flay backwards once or twice, and make other novice mistakes with him. But I let them make the informed decision.
The ones that yell at me with the "NO SONA" crap, which happened earlier this week actually, I have no desire whatsoever to accommodate. I have a fairly crazy work schedule, and most work days, I can only play 1 match, 2 at the most. If they cannot speak to me with common courtesy or even a modicum of respect, then I see no reason why I should give up my one game time slot to play the champion I love to play. I won't be rude to them, I won't say anything at all to those people. For them, I simply lock in Sona, and /mute them as soon as the match starts. It might be a little immature of me, but I don't really trust myself to say something to them in chat that wouldn't come across as condescendingly chiding, so I don't try to say anything in champ select. If I could think of something to say that wouldn't make me sound like a pompous asshat, I might try it, but I've yet to think of anything, so I just stay quiet. If you've got any suggestions on what to say to people like that, I'd love to hear them!