When absolutely necessary, my work-around is to post the handounts somewhere for download and then edit the automated confirmation and reminder emails, especially the subject line. You have to train your users to look for this.
This was rare situations where the presenter was providing information a few days in advance, but he didn't want it shared with the world. Of course, you can always download the registration list and manually email; but then you always have the late additions.
For instance, I might add to the subject line, appending "AND HANDOUT INFO" and then add instructions in the area where you can customize text. I had one where handouts would be ready in advance, but not until a few days before the session. No formatting is available for this text, but all caps can help. So I'd add a section.
Most people don't actually READ the confirmation and reminder emails. I'd stress in the invitation emails that the handout info would be in their reminder messages.
Be careful about then using that webinar to clone the next if you don't want that custom text to transfer.
