Marc,
You could always suggest that members of the coalition put their plans in writing, and everyone sign their names to the document. That should provide at least some disincentive to renege. Another precaution is to make sure things are set up so that libertarians have enough control over the event so that if ANSWER does deviate from the agenda with speakers talking about other issues, etc., that libertarians are in a position to respond in kind by promoting other parts of the libertarian agenda. You could even seek agreement up front that this would be an appropriate response if one coalition partner violates the understanding.
Love & Liberty,
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