"How would I go about doing this? Is there something similar to OperationContext.readResourceForUpdate that would allow me to update the parent model from a child handler?"

This could be done from AbstractDiscoveryAddHandler.updateDiscoveryOptionsOrdering(). You call context.addStep() there. Your "writeHandler" param there does not need to be the registered handler for the write-attribute op. It could be an inner class. The "writeOp" param is important though. The "address" field in that param is what determines what Resource is available for update when the handler executes.

If you decide to just allow users to write this attribute, then none of this matters. Otherwise, Tomaz raises a good point -- the management API metadata will say this attribute is writable, but it's actually not. So in that case the attribute should be registered as read-only and the update should be done via what the mechanism I describe in the previous paragraph.