'Color > Black & White': add option to adjust threshold

Converted to bug report (wishlist) from question 246534:

This is more of a feature request than a real question, but I have failed to find a more adequate place for it... please let me know if I should post this elsewhere.

The "extensions"->"Color"->"Black and white" transforms a colour drawing in balck and white (not grayscale, but back/white only). It does it by computing an "equivalent gray value" (following some convention) for each color, and then setting to black everything darker thana 50% grey, and to white everything brighter.
It would be very useful to have a simple dialog that allows the threshold (50%) to be changed.
Right now, I accomplished this by editing the python script that the extension uses, but ti's far from convenient... especially if you have to iterate to find the optimal value.

Thanks

Giacomo