I checked again and, perhaps more importantly, you can outright a player (as I believe was the case for Melo) only once without his permission. The second time, in the example I tried, he has the right to choose free agency. That would put a real limit on a team's aspirations to move a player on and off the 40.
This is true, but also, the waiver period is actually only 72 hours for players on optional assignment to the minors, like Melotakis was.
I still think that waivers might "reset" once a player is re-added to the 40-man, but it is mostly a moot point. It would really only impact a certain class of player (making a decent salary but with less than 5 years service time, like Milone last year, so they can't refuse the assignment without forfeiting their salary). And if such a player clears waivers once, he would probably be likely to clear again. So it doesn't really matter if the team can skip waivers on him the second time.