I did it 
I’m having a good day. (Every day that brings us one step further to get a momentary button is a good day 
)
I recently use a GP Script which increases widget values over time (to manipulate the filter cutoff while playing with both hands). For that I also use a “Reset” button that sets the filter cutoff to a default value. This button is supposed to be a momentary button too (I currently use the SetWidgetValue(0) method but I think it would be much better to have it as a widget option).