Hi,
Do not mix to things:
Cloning of the SnapMirror destination doesn't allow to change anything in the parent volume, yet you logically see a full, read/write accessible entity. So taking this further, if you change every single block in your clone, you will effectively create a 100% size capacity overhead. Or - if you wish to do so - you can split the clone earlier & you straight away have two physical copies: one read-only parent & the new read/write accessible child volume.
Bottom line: you can write to the clone, no matter whether you split it, or not.
Regards,
Radek