Hi
This currently isn't supported, but should be achievable by writing a custom implementation of the ChildCollectionValidatorAdaptor. If you want to create a work item for this on the issues page, then I'll
try and have a look at implementing this, but I may not get round to it for a couple of weeks. Alternatively, if you feel like having a go at implementing it, then feel free to submit it as a pull request on GitHub.
Jeremy