As you mentioned WebTexter was started mainly for Irish mobiles but also supports betamax providers which will give cheap but not free sms.

I gave a quick look at the one's you mentioned but couldn't find info on what I'd need to integrate SMSenger to webtexter. For freetext they use a captha seems to need javascript to work so would be awkward to integrate to webtexter. If anyone has information on the http transaction needed to send to them I could possibly integrate them.