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Yes you could remove the field stop in your eyepiece but the results wouldn’t be pretty. Firstly the field stop if positioned correctly at the focal plane defines the sharp black circular edge the you see in the field. Secondly you would see significant edge of field aberrations that the designer of the eyepiece wanted to control with proper placement of the field stop and selection of glass types, number of lenses and curvature of all surfaces. The wider the apparent field required, the more complex the lens groups and larger some of the lenses need to be.
The field stop of simpler eyepieces is normally a black sharp edged ring that you can see if you turn your eyepiece over and look into the barrel. Often it unscrews but shouldn’t be tampered with or you lose the sharp edge as any new position may not be at the focal plane as intended. On complex eyepiece designs the field stop is internal and you cannot remove it without removing some lenses first. NOT recommended.
Note Powermates and Barlows are different in that the Powermate lets the eyepiece behave exactly as designed with or without the Powermate in place and the only difference being the amplification factor. With a Barlow things are more complex and many factors contribute to a successful combination.
I remember a very long time ago I used a Tele Vue 24mm Widefield (65 deg.) eyepiece in a 2.8x Klee Barlow and noticed some obvious vignetting. The same eyepiece in a 2x Clave Barlow showed no obvious vignetting. I always thought it was the small clear aperture of the Klee compared with the very large, almost full aperture of the Clave but there was one other difference and it was that the Klee was very short and the Clave very long and the eyepiece exit pupil behaviour and eye relief was considerably different between the two. A Barlow extends the eye relief whereas a Powermate does not. If the eye relief is extended too far then vignetting of the exit pupil is noticeable.
I got kind of off topic a bit. I don’t have the binoviewer you have but I thought I’d share how I achieve higher powers with a set of Powermates (and Tele Vue T-ring adaptors) and a set of three focal lengths. I chose the 24mm, 19mm Panoptics and 16mm Naglers because of their form factor (volcano top shape makes binoviewing easier at shorter eye relief). I also have a pair of Tele Vue 8-24mm click-stop zooms which also work well. These are not volcano tops but apparent field of view is less and the eye relief is ample to see the entire view when binoviewing. (Haven’t used this combination for a while so going off memory).
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