From the letter giddeanx posted from Mr. Zipp, this line stood out (to me anyway):
You will need their help, for my messages will be encoded to confuse the enemy. Even now it is likely you need their help.
Jeromy has indicated in OOC responses that Mr. Zipp is a puzzle that once we solve it, he thinks we will know we have solved it, like we will see or realize something and it will be clear it must be correct.
The quote above made me wonder whether at that moment in the story, Maddison already had some hidden message or code - maybe something we missed. It might bear examining what she had already at that point in the story. It could be he is just talking about the coded pigeon messages, but maybe there is a hidden message within this very letter or elsewhere?
Also, the clearest references we have to "Zipp" aside from his letters are Skipper Zipp in the Dr. Seuss book and the "Mr. Zipp and The Letters" poster found in one of the drops. Does "Letters" here refer to messages, or to individual alphabet characters found somewhere or ???
Otherwise, Mr. Zipp refers to Maddison as an "instrument" and a "microphone" ... musical references ... and Zipp might be related to Zip guitars. (I know there are electric Zip guitars, but I'm not sure about acoustic). Should we be looking for a band or a music show of some kind?
"Skipper Zipp" may imply he is a leader of some kind. I still think the theory that Mr. Zipp is Jeromy and the ARG itself is his child makes a lot of sense, but I'm not sure that is within the bounds of valid speculation for this mystery.