Another approach to what Mearls seems to want PrCs to do would be 4E's boons system. Basically, mechanically, just treat memberships, and "achievement" type stuff as grounds for giving boons in place of similarly-valued treasure.
If you want to have bits of mechanics that the GM can grant to PCs for doing stuff, why not tie it into the existing system for bits of mechanics that the GM can grant to PCs for doing stuff?
If you want to have bits of mechanics that the GM can grant to PCs for doing stuff, why not tie it into the existing system for bits of mechanics that the GM can grant to PCs for doing stuff?