I'm against donations in a form of a fee. P2Pool has a number of disadvantages for miners comparing to centralized pools. There are large pools with zero fees. By asking money from miners we scare them further away. ASICs are expensive and have a limited useful life, so miners are generally only concerned about short-term ROI. Miners are not who care about future of Bitcoin. They are not ones we should expect donations from.
Instead we should appeal to those who have a stake in the Bitcoin world. There are large bitcoin holders and Bitcoin businesses. We should set up bounties, grants and subsidies directed towards mining decentralization. This way we can also fund P2Pool development.