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I don't think so, as there is no way for the iPad to request that it receive a screen from a remote machine - it's initiated from macOS.
 
I have been interested in this as well, but from everything I have read - no, it does not seem that this is possible. At least currently.
 
I'm hoping you can still VNC to your mini and initiate the Sidecar connection that way. That'd work for me. Even better if it's possible to assign a keyboard shortcut / macro to Sidecar. Wake up the mini with a keyboard (blind-type password if necessary), press shortcut and you're good to go.
 
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