Add custom files/file-fields to panels pages I would like to add content from a set of file fields to particular pages (via panels/panels everywhere/panelizer) with the ability to change the files by panels IPE editor or backend form. 

Ideally, I would like to just add a custom content pane, or minipanel (or whatever it needs to be) to specific panels pages, and have that widget be able to upload files that print to the page, and that page only. It would also be nice to reuse the widget for other panels pages for different sets of files. I'm trying not to involve a whole other set of node/content types just to do this.

Could anyone point me in the right direction? Thanks.

Detail: The purpose is to upload files and add file paths to an element on some pages which is the image/video source for a fullscreen image/video background script.
What you are looking for is a customized fieldable panel panes. You can also check Panopoly Widgets for a similar example. It contains a lot of custom widgets which can be embed in panel panes and customized.
Fieldable panels panes makes your widget data as fieldable entities instead of node/content types(which is what you want).

If you want dynamic background images on a specific page context, you could check the dynamic background module.