Exactly as you mentioned in your question, as Cognos Power Play Studio is accessed from the IBM Cognos Business Intelligence portal (IBM Cognos Connection), you can identify if it is whether deployed in the environment and how many users are accessing it by using the Cognos BI data.
In order to collect the necessary information from Cognos BI, you can use one of the tools that is available on IBM official website: http://www-01.ibm.com/support/docview.wss?uid=swg24034217.
However, the output triggered by this tool doesn’t provide a straight-forward mapping between functionalities identified in the system and the licensable Cognos products, therefore you will not see directly the associated functionalities to the Cognos Power Play Studio and its correspondent users. A mapping process would need to be applied in order to be able to link capabilities, roles and users in such way that you will be able to find out the final deployment usage.
Happy to contact us in order to provide more details about this subject.