This is an excellent idea. On a practical level, however,an angler observing barbel milling around over gravel in early June is very different to having "evidence" of succesful deposition and fertilisation of eggs. Will there be a way of filtering the reports to extract the real observations of actual spawning activityu, from other similar activities?
The previous comment regarding not disturbing the apparently spawning fish is also important. One of the main arguments for maintaining a close season on rivers is to allow the fish, nesting birds and river banks three months without disturbance. It is important that this survey doesn't encourage anglers to negate this benefit. As anglers, many of us would be tempted to take a bucket of bait with us and feed up a few spots, which could also be counterproductive.
I don't want to be a wet blanket, as I think this is a good idea in principle, but I can see some potential for unintended consequences.