I can say that this isn’t really the target use of Electron. I’m also not sure that it can be used in this way as it is currently designed. I’ll see what I can find out though.
Confirmed with the developers that this scenario isn’t supported and this isn’t a capability that the Electron team is planning to target at this time.
Electron could be forked by someone interested and the startup code could be rewritten so that the WinForms app would handle the window and building the frame that Electron draws in. But that would be up to the person or organization building that capability.