Hi Angelo;
PowerServer would have nothing to do with your iFrame interaction. The PS Server would only broker your App's data access. So the key question would be what PCI compliant API's exist to interact with iFrame from the PB App (whether it's C/S, PowerClient or PowerServer based) based on C++. Since I don't know iFrame, it can't answer that part of the question. Probably a good question for the iFrame folks to guide you further.
Now if your Web API's were built with SnapDevelop where you would be coding in pure C#, that could be a good question. Again though, the iFrame people should be able to guide you with a C# DotNet API requirements.
Nite that Appeon PB now supports direct C# DotNet assembly consumption in your PB App. So the same iFrame API might also suffice when called from your PB App as well. HTH
Regards ... Chris