I usually bring my guitar between SFO and JFK a few times a year. The best thing to do is to have it stored in one of the closets at the front of the aircraft. Just ask the FA right when you board. This will be possible on your ORD to HKG leg. It will save you from taking up overhead space or having to gate check it.

For the Embraer 145, I'm not sure. I remember having to gate check once on an AA Eagle flight just because the bins on those commuter jets are just too small. If you do have to gate check it, be sure to ask for it to be returned at the arrival gate in ORD instead of them checking it all the way to HKG.

Also, you should have a good guitar case just in case you do have to check it. I use a double arched hard case that is pretty durable. Be sure to loosen all strings, and a humidifier may come in handy as well.