Hi zapjb,
I'm afraid it sounds like you've eliminated all the options that I know of. The only way to send an SMS via GV without using data is when you're replying to a conversation that's already going on. GV would have to already have a forwarding number established for your recipient in order for GV to handle the message as a standard SMS. It is for this reason that I typically use the GV app to initiate a message, and then handle subsequent replies in Hangouts or another SMS app of my choice.
The only possible work-around that I can think of would be to use the Voice+ feature of CyanogenMod, but I've never used it so I don't know for sure if it would do what you want.