But most of the players who would have previously been traded in August can likely refuse a minor league assignment in August, I'd think.
Otherwise, I think the loophole you've found seems perfectly reasonable/unlikely. Because if the player has any value he wouldn't clear waivers (and therefore would never come off the 40 or go to the minors) in the first place).
If Rios ends up back on the Allentown roster and the Phillies want to trade him on August 2, why not? If they wind up outrighting Austin Davis and the Yankees (who would be way down the claiming pecking order) want him he will either fall to them or he won't, a back-door trade won't work if another team claims him and is unnecessary if another team doesn't.
Conversely, if the DBacks outrighted Greinke to see if anyone claimed his salary, and no one did, he'd refuse his assignment to the minors, Arizona would still be on the hook for all the money and couldn't trade him, and then Greinke could sign with any team he wanted. So, no angle there.