I know it might be cheating but could you get hold of a ready to use V-tail mixing model for your tx? That would get you there straight away.
That wasn't very clear of me. I meant - there must be a default tx programming file available somewhere with V-tail mixing taken care of. I'm not an expert in the slightest in programming that tx but I thought it might be a shortcut worth tracking down

Getting a physical mixer might be a bit silly as you say.