Answering (somewhat) my own question here, in case someone else is looking for the same thing.
So it seems that unfortunately no such reader exists for iOS (perhaps there is room in the market for an enterprising iOS app developer?) There is however a very workable solution for Android in the form of this app:
Search in Books.
While not a reader per se, this little app does exactly what its name says, i.e. it searches across multiple (and individually selectable) ePubs and PDFs for specific words and provides a breakdown by book, with some textual context around each instance found. Individual search results can either be expanded for more context in plain text, or viewed in the original ePub of PDF formatting.
This might not be sufficient to everyone, as it is rather a search tool with integrated viewer than a full-fledged e-reader app; it is also an old, unsupported application which has to be sideloaded as it is not available on the Play Store. But it is perfect for my use case of searching and displaying recipes.
Hope it helps someone else!