Thanks both of you for taking the time to answer. Firstly, just ignore the drums - I haven't done any proper work on those yet - I just know that the overall feel is what I'm after, and I'll refine/add the extras in for that later. I've just been focusing on the instrumentation for the chorus for now.
Bernd, I do agree that it's quite close - some people I've talked to agree with us; however others think the chorus is seriously lacking.
Neil, if I took out the harmonics from the verse, I'd be left with just a bass guitar and drums, and I'm not the minimalist type. Also, the harmonics is the basis of the introduction to the song (which of course you didn't hear), so it's the guide for the journey of the song. My original experimentation with power chords seemed to gravitate to a sound which occupied the same space as the acoustic guitar rather than contrasting. However, just in the past hour, with fresh ears, I've found a better sound and avoided the clash by moving the power chords up the fretboard. For the middle 8, I've went with root/3rd power chords to make the major/minors clear. So, well done, you win the (non-existent) prize.
BTW, I'd already tried everything I could think of to fill that space, but I didn't want to pollute any ideas - I wanted to find out what direction others would go in so I could narrow my focus. I tried choral backing, orchestral strings, synth pads, organs, pianos and many others. After a while, everything sounds bad, so it's sometimes a good idea just to leave things and go back fresh later.