I don't think you can capture input in Power BI, I haven't seen any visualisation where you can get a user to input data. You can certainly use Power BI to report data, and you can utilise the reporting capabilities.
Furthermore; I'd be interested to know how are you thinking or planning to utilise Power BI for the planning solution? With PBI in Cloud, I'd imagine there would be a sync lag of 30 minutes to 60 minutes for the data to synchronise from your on-prem SSAS into PBI Cloud. Given planning solutions are expected to churn out reports almost instantenously during the planning period, this might be another factor.
Lastly, I'd be interested to find out about your planning application as we are using an off-the-shelf planning platform. I would be obliged if you can share high level details, requirements & features of the solution.
Thanks
Kaz