Id be interested in a cup like this, that being said;
There is an objectively tiny pool of players ive seen play at a higher level, be it even main adv or invite, that have gone down a division or more and played with people who werent a part of their immediate friend group/circlejerk; and the amount of players who have done that and not wanted to blow their brains out after a week is even less. Playing with people who play a lower level than you can be very frustrating, so I'd keep it in mind when proposing something like this.
Also, 36 players are too few. How are you selecting who gets to play and who doesn't? im sure there are definitely more than 36 players interested in a unique experience like this. Are you excluding people who have played invite? Id argue there's a huge gap between low invite and mid/high invite.