It's been nearly 2 years since this topic was created, and now I maintain a series of patches called gtk3-classic ... previously known as gtk3-mushrooms.
Even though my C isn't great, I have managed a couple of fixes - including the ability to scroll over tabs like we could do in GTK2.
It is packaged for Arch (which is pretty straight forward, actually), but I haven't forgot about Ubuntu MATE! At some point I will have another go at creating a PPA / Debian package so you can enjoy classic features of GTK2 in GTK3. I just need to figure out how to "turn off" tests for the Ubuntu gtk+3.0 package.
I think patching GTK3's quirks is a good workaround until STLWRT enters the scene as a possible GTK "slot in" replacement. 