Not something I've tried, but perhaps a filter will do it. Filters preprocess html to change the output. For example, taking a url and making it a clickable link. You may find there are some filters that might convert a url to an iframe e.g. a youtube url will be converted into an iframe, so you would need to run your filter after that filter.
General developer forum
Data privacy: Altering markup of src attribute in all occurrences of the iframe element
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