Is it possible to use the cached RSA key while using the .NET assemblies instead of using the WinSCP.sessionoptions parameters?
No. See https://winscp.net/eng/docs/library_from_script#default_config
[quote]I can't have the key hard coded in my script or an xml file.[/qoute]
Why? How's loading a host key fingerprint from a registry better than an XML file? And if you prefer the registry, load it from the registry.
See also https://winscp.net/eng/docs/library_example_known_hosts