A full-scale example app is planned as a deliverable for modular hivemind. Most likely the example will be a wallet app.

I don't expect 2nd layer data to have any major impact on the chain. From a bandwidth perspective, RC limiting will prevent traffic congestion. Most of the computation takes place at the 2nd layer in this model, and the design is such that not every modular hivemind node will need to run every 2nd layer app (for example, in the case of the light wallet, users could just run their own local copy even). This means that each modular hivemind installation gets to determine what data it wants to keep, and it only needs to keep the data for the app or apps it wants to support.

There's no overall limit on 2nd layer data other than that imposed by RC limits, but there is a limit on the size of a single 2nd layer transaction (currently it's 8K per transaction, IIRC).