You're just saying the same thing in a different way - you're looking at the key of the harp as "I"; I'm looking at the key of the piece of music as I, which makes more sense to me (although your "vision" seems to be the more common one). So if the piece of music is in the key of E (I), you will use a harp in the key of A (IV), to play cross-harp.

Yes, you can stand that on its head and say if the harp is in the key of A (I), then you can play cross in the key of E (V). I don't see how that's any more helpful though. In fact, it strikes me as unnecessarily complicated; for instance, if someone on the bandstand tells you that the next song is in Bb, so that Bb is the I chord, then you have to think of Bb as the V chord in order to count down and deduce what harp to use, but then think of Bb as the I chord again when you're figuring out the chord changes as you play ... Of course, once you've done this enough times, you know automatically which harp to use, but, as I say, it seems unnecessarily complicated for neophytes.

- thurg (who has never played harp and knows nothing about it)