How to use carrierwave's from_version to reference the processed base image, rather than the unprocessed one - carrierwave

Question

I am working on a site which needs to have a newsletter page, with thumbnails. The PDFs are stored on a different site, but that site does not have thumbnails, so I must generate them myself and link the thumbnail to that site.
I cannot store the original PDF — the linked version is canonical, and having out-of-date cached versions is unacceptable. The additional storage usage might also cause an extra monthly expense for the non-profit organisation the site is for, so I do not want to leave these lying around when they're not needed.
I have the following code:
class DocumentUploader < CarrierWave::Uploader::Base
include CarrierWave::MiniMagick
storage :file
def store_dir
"uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
end
def content_type_whitelist
'application/pdf'
end
def full_filename(for_file)
parent_name = super(for_file)
return unless parent_name
extension = '.pdf'
base_name = parent_name.chomp(extension)
if version_name
base_name = base_name[version_name.size.succ..-1]
end
[base_name, version_name].compact.join("_") + '.png'
end
# Process files as they are uploaded:
process convert_pdf_front: [500, 500]
version :medium do
process convert_pdf_front: [160, 160]
end
version :small do
process convert_pdf_front: [80, 80]
end
private
def convert_pdf_front(width, height)
manipulate! do |img|
img.format('png') do |c|
c.resize "#{width}x#{height}"
end
img
end
end
end
This works as expected, but the medium and small versions are converted from the original PDF, making for a rather slow process.
If I replace the processing code with this:
version :full do
process convert_pdf_front: [500, 500]
end
version :medium, from_version: :full do
process convert: 'png'
process resize_to_fit: [160, 160]
end
version :small, from_version: :full do
process convert: 'png'
process resize_to_fit: [80, 80]
end
then the medium and small versions are converted from the full version, but the original PDF is left lying around.
Is there a way to use from_version to do the conversion from the converted original document — that is, from the 500x500 PNG rather than the PDF — that doesn't leave the original PDF cluttering the place up? Is there a special version name that points to the original document after it's gone through any process commands, rather than before?
(I would rather not simply have a reaper process nuke the PDFs later, as Carrierwave regards those as the originals. I'd like Carrierwave to treat the 500x500 PNG as the original in all cases and never store the PDF once it's done with the conversion process.)