IJ2 plugin development: don't show imageDisplay choice when variable is initialised manually

I have an ImageJ2 plugin that operates on an image. It declares
both an net.imagej.display.ImageDisplay and an ij.ImagePlus input
parameter that the scijava loader populates with a reference to
the same active image.

Now I would like, in the case where no image is open when the plugin
is started and those parameters initialise to null, to open an example
image from within the @Plugin initializer and proceed with plugin
execution using that image. This appears to work by manually
initialising the two plugin parameters mentionned above, pointing them
to the newly opened image. However the plugin dialog then shows two
dropdown choices for these parameters:

These dropdown menus (for Imp and Display) are not shown if an image
was already open when calling the plugin. Is there a way to prevent
the menus from showing ? Is there a way to simulate a fresh plugin
start from within the @Plugin initializer once the example image is
open ? Is there a way to cause the @Parameter annotation parser to
start all over ?

The source code leading up to my issue is the few lines near the beginning of the paramInitializer method here:

[side-note: each menu contains four dropdown entries pointing to the same image (twice through the title, twice through the path), indicating that I might be doing something wrong with the initialisation; it may also have something to do with
Converting a Dataset to ImagePlus implicitly shows it