A hopback is definitely on my wish list. I haven't done anything about it as yet because I figured you must have a CFWC and pump to get one to work properly. But I guess as long as it was planned thoroughly, you could probably get something to work in a gravity based setup - transfer the hot wort from kettle into another bucket, through some kind of hopback arrangement, and then chill the wort in that bucket with the immersion chiller as per normal. Sounds like a lot of effort, but from what I gather from the reports, a hopback makes enough difference to make it worthwhile.