I've got lots of pages within a dashboard and would like to add the same filter to each page. I know of the ability to enable 'Sync slicers' but that's not exactly what I want to do. Is there a way to enable a filter at the highest top-level page that will be applied to all reports or pages that are on that page?
I have to admit that I do not understand what you are aiming for.
Basically it's not possible (at least not now) to change a slicer on a dashboard (the naming is important) to alter a filter, and this filter will be "propagated" to any report, where a visual used in a dashboard originaes from. Visuals on a dashboard can originiate from different reports, from the same report but different pages or just display two visuals from the same report page.
Currently it's just possible to synch slicers across different pages of the same report.
So maybe you might consider to create a "top" page that contains all the filter, that are synched to all pages, and use bookmarks to navigate to all the pages of that report.
Actually, the "Sync Slicers" provides more flexibilities. You can decide which pages to show or to sync. If you sync all the pages, the slicer is the highest top-level. So what's it that you expected? Or you can consider the Report Level Filters. Please refer to the snapshot below.
Dashboards are not designed with User Interactive filters in mind.
The only method of using a slicer within a dashboard is to create a Report Page with the slicer and the dashboard elements you want, then use the "Pin Live Page" option.
Could you please mark the proper answers as solutions?