How to
1: Add a file browser to a a UI panel to get user selected file path & assign to var.
Useful: How can i open a file select dialog via python to add an image sequence into VSE?
2: How to have button that executes a specific part of a script.
(Copying code from Blender's interface meant that the filepath that I ended up using was one also used by Render>Output.)
Best source I have seen in 4 days of searching https://blenderapi.wordpress.com/2011/09/26/file-selection-with-python/#comment-4
Useful: http://wiki.blender.org/index.php/Dev:2.5/Py/Scripts/Cookbook/Code_snippets/Interface
bl_space_type='SEQUENCE_EDITOR' bl_region_type = 'UI'. This should be a great starting point. – p2or Dec 06 '15 at 11:07