Hi Uwe,
your report on Barnard's Nebula several years ago actually started my own interest in this object :-)
It also works with eyepieces with internal field stops. Of course, you have the open the eyepiece to place the bar to the field stop. In fact, my 6mm eyepiece with the black bar is a Skywatcher 6mm UWA that I did not use anymore and that I optimized to reduce stray light with an internal baffle. Here are two images, it's the middle one in the upper picture, while the lower one shows the additional internal baffle above the negative element (the occulting bar is not shown).