Unfortunately, I'm not in a position to help at the moment, but do have a thought on this approach:

for users who can prove they are subscribers by uploading the latest
installation files. If the uploaded file matches what's on the server,
then their browser gets a cookie that allows them to screen

At present, the data update file (not the install...just the update) for VL is about 9 MB; for VL+, it's about 20MB; and for SI Pro, the install weighs in at about 65MB.

Any way you look at it, that's a lot of data to upload...particularly when most folks are on asymmetric broadband, where the upload speed is a lot lower than the download speed. If I had to guess, it might take 5-10 minutes just to upload the SI Pro file for a single user.

Might it be easier and quicker to have the user generate and upload an MD5 checksum of the install files as a mechanism for authenticating that the user has the latest install?