I think South East Asians Servers needs to be compatible with their Internet Service Provider per location or region that's why only local games are mostly played in Garena. I don't also think Battlenet itself in DoTa will be helpful. I think Ranked Gaming Client can considered here since they are allowing every internet service provider in their client. Just don't know if today it's still operating or not.
Never heard of Hamachi.
I see, I didn't know garena has some sort of ISP problems. I usually play there, even till now, with my friends who are within the south east asia area like singapore and malaysia. We haven't encountered much problems with it. Regards hamachi, it is a program that links our networks like we are in the same Local Area Network (LAN). Which is nice because Warcraft III uses only LAN.