I'd be willing to help! But I'd also suggest to just make it about the configurable setting and leave the rest to the user. I think signalling about blocksize has to happen out-of-band for the time being. Because it is potentially a lot of code complexity. And simple IMO beats complex here.

Just make it mandatory to start bitcoind with -maxblocksizelimit (or similar) and have an edit box for bitcoin-qt that has to be filled with a value. The amount of code change should be about the same as BIP101.

Start requesting this value at some switchover date in the future - maybe at the beginning of Gavin's increase schedule. Reason for this: Time for user education on building a function Bitcoin network.

What do you think?