Yes, I understand what you mean.
I have no idea how complited something like this would be. I will forward this suggestion to starkad, but seeing as I gave him important things he needs to code months (or literally years) ago and he has not worked on them, I would not keep my hopes up for anything that requires new coding....
Though I am not entirely sure this would really be of much use: There might be users who would not buy a set if they only liked the hair of a set and would buy a pak with all hair instead - so that would be a win. But there might also be users who would buy a whole set just because of the hair (After all the Treasure Trove used to only sell single items at the beginning and people were willing to spend a Fairy Coin for just one item), so might end up buying 2, 3 or 4 sets because of that hair - in which case they would, should such an option be implemented, only buy one pack in oposite to 4 sets. Do you understand what I mean? It might cause some users to spend more Fairy Coins and others to spend less and overall we might end up in the same situation.
I think the whole main problem with lack of donations is the lack of active users and the only real long-time solution to our problem would be to gain new (active) users instead of trying to find new ways to "squeeze out as much money as possible" of the few active users we have.

I am quite hopeful that most people who cares about this site understands that they are not just "paying to get some pixel", but that they are donating to keep this site and the overal experience going and probably most users who habe an interest in this site continuing to exist are already doing all they can financial wise. But it's just not enough when the whole financial burden is resting on the shoulders of only so few people, and sadly at the time mostly on starkad and me.
It's not that I think finding more ways to motivate people to donate wouldn't help at all - Like, for example, out of the over 600 people who saw the latest MT announcement so far only 6 users decided to donate. That's a really lousy rate, but the remaining 594 users are most likely mostly users who don't care enough to donate no matter what, or simply don't have the means to donate and the part of users who don't like the set or any of the donation perks available - and these are the only fraction out of the whole pot that possibly could be motivated to donate by other means. So, no matter what things we add to motivate users to donate: We would only reach a small amount of "potential donater" as most users are the ones who never donate or would donate in all their life. So, again, the only long time solution would really be to get new active users.