I am a bit unsure how Zeppelin plans to support both 0.4.x and 0.5.x lines of Solidity, because clearly some form of support for 0.4.x seems to be needed in the short term given the stability of the ecosystem around it.

Here we've received an indication to use the solc-nightly branch, but I think that is just an intermediate solution.

I do not have any proposal at hand, but this may be a good place to discuss this in the public. Please delete if you think otherwise.