LRCloud users will not be able to create a custom SmartResponse Plugin in their LRCloud environment. If they have an old on-prem LR system, they could create the SmartResponse (SRP) and then export it. However, it is not often that a customer has a spare LR environment and will need to open a support case for assistance. Any internal LR employee (Professional Services, Support, and Sales Engineers) can create custom SRPs in their personal test environments, export them, and then provide the final .lpi file back to the customer.
Since the Client Console server in LRCloud is configured for Security Technical Implementation Guide (STIG), users can only access file types with .lpi extension when using the "Create Plugin" action. The SmartResponse Plugin Manager does not allow the user to select any scripts or the actions.xml that will make up the custom SRP.