Is there a way to mount or expose a folder within a package to another folder? I'm trying to create a time-lapse video from jpegs using GoPro studio on the MAC but that (and alternatives I've tried) don't seem to understand how to access all the photos in the Photos app.

If I right-click on the Photos app and go to 'show package' contents, I can see all my photos in a directory called 'Masters'.

Is there any way to mount or otherwise expose that folder as a standard folder that other applications might be able to work with?
You can create an alias outside the bundle to expose this folder (by command-option dragging "Masters" out of the bundle). However, it is not always advisable to mess with the contents of a bundle, so proceed with caution. If you're just reading the photos, there's no problem, but Photos may not take kindly to modifications. The "safest" (but most annoying) way to access items in your Photos library is to export them from Photos itself.