Would is be possible for a developer to create a completely transparent sidechain that is pegged to Monero?
That would certainly be possible. To prevent issues with the mainchain we would have to implement a cryptographic proof, i.e., the amount of XMR that went into the sidechain would have to be equal to the amount of XMR that goes back into the "mainchain". The biggest issue with sidechains currently is how to secure them. One would need to convince miners to secure their sidechain. In sum, it doesn't matter whether the sidechain would be private or (completely) transparent.