The Reader View is right now accessed by using Firefox' internal ‘about:’ protocol. Currently there is neither a way to work on pages behind that protocol, i. e. redirecting to such a page, using WebExtensions nor exists a specific Reader View API.
The latter, however, might get implemented as announced on the respective mailinglist: https://mail.mozilla.org/pipermail/dev-addons/2017-August/003063.html The corresponding bug, to which the lowest ‘P5’ priority is assigned, can be found here: https://bugzil.la/1286387
Once the functionality is available, I plan to update the add-on.
[Update 2017-11-20] Add-on has been updated to work in Firefox “Quantum”.