Well, I would say 'an herbaceous', but that's because I don't pronounce the h. If you do pronounce the h, the 'an' still seems wrong to me. Edited to add: I realize that your whole point is separating the sound from the spelling, but the choice between 'a' and 'an' is all about sound, so I'm not able to agree that you can separate them.
And I offer another page: https://stancarey.wordpress.com/2013/11/19/an-aitch-or-a-haitch-lets-ear-it/
That's pretty much it! I'm a big fan of audiobooks for doing chores, or travelling, and the amount of times I've listened to books that I have also read, and am surprised by the pronunciations of the names, that, in my head, are WAYYY different. And the same goes for listening to authors reading excerpts - the way they say the name can be totally at odds with how I read it in my head. But that's part of the joy of reading some Genres, isn't it?
