Adam Silver has been pushing this like a prostate exam since he took over the league. I don't see anyway Kawhi and Embiid, for example, have any interest in taking part. There are already a fair amount of players who don't have much interest in the regular season, saving their bodies for the regular season.
On the other hand, Summer League has really exploded in both quality and interest over the years. They should really take the cue from the Larry Bird and Magic Johnson awards and create a Nate Robinson Award. Little Nate (oh that nickname, how I hate) made the summer league what it is today.
An in season tournament that I would watch would be in January and only open to guys on rookie contracts and on their G League affiliates. I don't see a million dollar purse really motivating the half of the league already making 300k a game, whether they play or not. Give the winning team the 15th pick, or whatever it is the one immediately after the lottery.
Fans will be excited to watch a lot of the younger guys who don't make the regular rotation, players would be motivated for a purse that substantially improves their incomes, and teams would be motivated for the pick. Hold the games in Vegas, LA or some other warm weather locale and some vets would have a reason to attend. The oft- injured crowd would be excited for a two week break. I think a lot of the G League guys would be motivated to play hard with more exposure. Especially the Kevin Knoxes and Cam Red dishes of the league would like to have a chance at being the man.
You will still get guys like Zion assured of getting a huge payday dropping out, but I think a million bucks is still something Scottie Barnes and LaMelo would be interested in.