Although not documented for /privatekey command-line parameter, it can accept an actual key data, not only a file.
The syntax is documented for -privatekey switch of the scripting open command.
https://winscp.net/eng/docs/scriptcommand_open#privatekey

Though I must say that I do not understand most of your post. So I'm only hoping that I've answered that you have asked for :)

Hello,

I am trying to use WinSCP as a launcher for secrets on my Delinea Secret Server solution.
I want the launcher to be proxied via Secret Server and I want the connection to be established via username/private key authentication, however the process argument used to launch WinSCP expects a file while Secret Server has the key embedded within the secret's fields as shown in the attachments.
And when I launch the WinSCP launcher the content of the private gets printed on the WinSCP prompt instead of using it as an actual private key.

Is there a way to achieve this?

Thank you in advance.