Sieve scripts might let you do what you want. It will run automatically upon the server receiving the new message. Think of it as server side message filtering.
You might be able to use a redirect command in a Sieve script to copy a spam message from the honeypot to a spam folder in another account if you use plus addressing to specify the destination folder as part of the other accounts email address.
I had hoped you might be able to use the fileinto command instead but that doesn't seem to support any way to specify a folder in another account.
See
https://www.fastmail.com/help/technical/sieve.html and forwarding messages in
https://support.tigertech.net/sieve
See the writeup on plus addressing in
https://www.fastmail.com/help/receive/addressing.html