I really like Matts suggestion on how to do a battlereport but would it be "legit"?
I have another suggestion, which is that we actually formalise this process. Irisado mentions playing lots of games and having reports on them to back up changes and suggestions, but if you view the majority of battreps they are simply a record of what happened and who won (90% by the victor, since there seems less incentive to write reports of losses

).
What might make this even more useful would be if we had a simple, set questionnaire for the players to fill in after the game. I'll write a few questions of the top of my head to give an idea what I mean:
[] Did any of the test-list's units appear overpowered?
[] Did any of the test-list's units appear under powered?
[] Would you change the costs of any units?
[] Did this list appear substantially better than the primary army of its 40K race, or simply a different spin on the army?
[] Was it clear that the test list had weaknesses to offset strengths, and vice versa?
[] Did this list play like a one-trick-pony (all air assault, all amour, all CC etc)
[] If yes, was that thematically accurate to the army's background?
[] On a scale of 1 to 10, how happy would you be to see this become and official army list?
Those are just the first questions that occured to me, but something like that could be a lot more useful than 'New Nids played Eldar. We ate them all, 3-0'.

Somewhere someone told me you have to do a action by action summary but if you read the NetEA Charter it just have to be documented...
Another big issue for our playgroup is that the opponent need to be playing an approved list, is that still necessary?
If we could do a battlereport in the style suggested by Matt and allowed playing against other development list my group could submit a lot more reports. (Right now we need to take notes to track the activations and it kind of takes away some of the fun from the game...)