FWIW, calibre's container class does contain a method (container.iterlinks) which can be utilised to achieve this. The container class is under constant development by Kovid and contains many other methods which make epub manipulation less onerous and more reliable.
If 'Modify Epub' was modernised to use this container class, rather than its homegrown equivalent (which pre-dates the newer official version), then I suspect future plugin maintenance/development would become easier in the long run. Unfortunately that modernisation would probably not be a trivial exercise ... and I'm certainly not volunteering to do it
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