If a participant can customize the coinbase, how would that be done?
Pool participants make certain decisions about how they mine. Some of those decisions are apparent in each block solved by that participant. The entire bitcoin community could provide extra rewards to one of that participant's bitcoin addresses if it could be positively identified.
A pool participant could compute the hash of "My bitcoin address is 1..." using their address (and any hash algo they want). Could they put the last eight bytes of the hash into the coinbase? If they solve a block and they know that there are bitcoiners willing to provide them with extra rewards, then they can prove they made that block by providing the "My bitcoin address..." message and the hash algo used.
Then I could donate some of my bitcoin usefully, and relatively anonymously, and so could everyone else.
This mechanism would help decentralize mining decisions (at least those that pool participants can make) in favor of those with bitcoin who are willing to push their agendas and help individual miners. Individuals could then send extra rewards to block creators whose blocks reflect desirable decisions.