But in the end, it doesn't really matter how many coins you have, the idea would be that if you have a stake in it, you should be motivated out of rational self interest to keep the network decentralized (as much as possible at least). Donating, say, around 0.1% of your BTC worth to a cause like this seems reasonable to me.
So, say if you're sitting on around 100 coins, that means donating 0.1 coins. At a total of around 1000, 1 coin. I'd expect we could get a pretty decent amount of donations together like that, if people are convinced it's a worthy cause.
That all sounds great. I would donate maybe 0.5% - 1% of my stake to some concrete p2pool improvement proposals, that had been suggested (and voted on) by miners and GHash users. I would not donate any directly to miners though as I don't see that being a long term solution.
It probably bears waiting to see about this scalability issue before actually soliciting donations though.