There is no way to do "texting" via wifi and not get charged a fee if done overseas. "Texts" are wired to go through a wireless carriers network to reach their intended person. "Messaging" like gtalk, aim, yahoo messenger, liveprofile.etc is strictly data. They will not incur a text charge if done on wifi.
PS. Be careful with WIFI calling. It is not a true VOIP. You WILL incur roaming charges if you use it overseas because will it is true you are using the feature, at some point your call has to connect to a wireless carrier in that country to make your phone call go through. I know this for a fact because I got nailed by TMO with overages when I made long distance international calls. I explained to them that it was a free feature on my G2x as explained by a sales rep at a store.....ahh but here is where they get you. It is free to use but the calls are still routed through them, using your minutes and their service. So I got hammered with long distance charges. Talk about misleading consumers. They sell you on "oh you can make any calls you want for free, even overseas." So be careful using wifi calling overseas as you will end up paying a pretty penny if you do.