I assume this cannot affect the prioritisation of the Coinbase transaction that constitutes p2pool miner's reward payments? Can you clarify this?
The coinbase tx itself? No (it would be in the block you solve, nothing can change that).
Spending the outputs of the coinbase tx? Yes.I'm not sure that the bolded part is correct. It depends on Luke's patch implementation, as well as the protocol rules.
Do multiple inputs to the same address constitute address re-use in the same way that multiple outputs from a single address do? LukeJr does indicate that this initial version of his patched client isn't the most aggressive. If inputs are being relegated, would it be possible to make this more subtle, and exclude inputs from CoinBase tx's? I'm deferring to Luke again.