I know its probably sacreligious to suggest.... but you could always purchase yourself some pre-isomerised hop extract from your LHBS and adjust your IBUs that way.
little bit of extract - taste - maybe a little more - taste..... that way, after you water it down, you can still have biterness just the way you like it.
Of course, you will still be diluting the flavour and aroma, but the IBUs would be ok and you could dry hop it to compensate.
