Bertrand, I may have a workaround for your issue. I have written Supertool scripts that can convert the ASSO information to event references which Gramps supports.
So instead of for example
you will get
(the example is from the Heredis sample family tree). So essentially the ASSO object is replaced with EventRef and the direction of the references is reversed. Does that make sense to you?
Please let me know if you want to proceed with this experiment. I can send the scripts to you.