Have you ever considered a single/multi tier of special labeling for those who have donated?

Since the online list is used to identify those in your family, you could do this via the locals listing, as the only change there is in bold lettering.

Assigning a special color for those who donate (buy credits), or tier it to a few different colors based on amount donated.  Listing the donators in locals in gold (or if you go with a tier, by the color for the range they are in).  Perhaps even labeling them as a donator in their profiles (or what tier of donator if tiers are used). 

Nothing fancy, just a small way to make those who donate be recognized.  If it's even a slight motivator for people to make that donation, it could be that little push to do it.