I used to be a part of one of the largest alliances, and I know the problem. However, I don't know what to say here.
In every factions farming guilds, there are people not farming, and pretending they do. We used to announced in the alliance chat whenever you donated factions in my last alliance. Even without a log, I noticed some people lied about - I could see that in checking the amount of factions of the guild. A donation log would get rid of this hypocrisy. At least, the other members of the guild would see who is really donating a lot, and who's donating nothing.
But on the other hand, factions farming guilds (and such a feature would be designed for them) are about... faction farming (sorry for this one). It's already very oppressive sometimes - many a ff guild leaderships see their members only as a ff bot, and don't care about you: they just care about the factions you donate. Having such logs would make it worst.
So I don't know - I won't sign for or against this suggestion.
EDIT - On a side note, I would laugh my *** off if I had the donation logs of most of the ff guild leaders I think
