In Firefox this is implemented as about:reader?url=…, and you can use that URL to load reader mode even on pages that aren’t detected as compatible (so that the reader mode toolbar button doesn’t appear).
This used be the URL on Firefox for Android as well, but if I recall correctly (I seldom use reader mode) it got changed in the Fenix world so that it’s a moz-extension:// URL.
I used to enter about:reader URLs all the time for pages that Firefox Android wouldn't show the button for. Do you know where the moz-extension:// incantation for this is? I would think it's a constant UUID, so maybe I just need to page through the source.
There really should be an option for "always let me try Reader Mode."
This used be the URL on Firefox for Android as well, but if I recall correctly (I seldom use reader mode) it got changed in the Fenix world so that it’s a moz-extension:// URL.