I don't have time to test this this morning, but here's a thought: What is the default setting for a newly created parameter? If the default is not to prompt, you could write your script so that it captures the existing parameters, deletes them, then recreates them. A bit of a work-around, but that might do the trick if the default is no-prompt.
Also, we might be looking in the wrong place for the prompt True/False. We assume it is associated with the parameter helper object, but it may be a keyed stream property.