Passing Dynamic Checkboxes to Confirmation Page

I originally had a form where I manually entered some PDFs that you could choose to download. Some of the PDFs required authorization before they could be downloaded, so the Confirmation page handled that (based on which PDFs were requested - data that got sent by using the "Pass Field Data Via Query String") component of the Confirmation Page settings.

NOW, I'd like to make this automatic - we're using a downloads plugin, and whenever a PDF is added to the downloads plugin, it automatically appears as a checkbox item in the form. This is working perfectly, and the data gets submitted to the form entries. The problem, however, is that I'm no longer sure how I can send which PDFs were downloaded to the Confirmation page - since I can't manually choose them from the "Insert Merge Tag" dropdown. Is there a PHP action/filter I can use to do this?

Thanks for your reply.

Is there an email address I can send the form URL to? I'm not sure it'll help, though. All I really need is to have the confirmation page be aware of which checkboxes were checked in the form.

I am indeed using gform_pre_render to populate the checkbox field items (I just pull the list of Downloads, and insert them into the $field['choices'] array - the "value" being the download ID, and the "text" being the name of the download file.)

Pre submission filter seems like it would be useful for changing the id to the filename, but I'm not sure it'll pass the information to the confirmation page.