This is no good when you want to use the fastmail domain for the aliases. Ever since the twitter @N theft[1], I've learned to never use a custom domain email when signing up for anything. It just increases your attack surface. I prefer to leave it to fastmail to protect their own domain from hijacking.
You could forward all mail from a Gmail address to Fastmail, and then use username+service@gmail.com for each service you use. Might be considered security through obscurity, but if nobody knows your actual email domain except for the Gmail address, less likely to have it attacked. Goes without saying to protect that Gmail account at all costs though.
I used to manually create them, then I learned of this and it's been a time saver.